I: What is Ninja?
As popular as Ninja may be in our modern society, not much is known about them. This is due to the fact that Ninja were an organized intelligence agency that operated covertly in espionage, assassination, counter-intelligence and unconventional warfare. In a way, one could see Ninja as a prototype of the American CIA or the Russian KGB. Such was the nature of Ninja that their activities remained almost exclusively invisible for the general public. In fact, activities of Ninja were so secretive that no one knew who the Ninja were, and a Ninja whose name is most known to us is by definition a Ninja who did not do his job well done. In other words, the most skilled Ninja were the ones whose existence is not even known to us. Hence, it is not hard to understand why they did not leave any manuscripts, and thus scholars of Ninja must work with historical records left by the government officials and oftentimes secondhand account provided by the offspring of Ninja families, many of which were written three generations after the period in which Ninja were most active.
During the Warring State Period, i.e. Sengoku Period, in the 16th century Japan, feudal lords were raising their army to conquer the lands and claim the country. Warfare in Japan was particularly advanced compared to that in Europe at the time both in the number of soldiers employed and in the preparations for the provisions. For example, the only countries in the world that could afford to mobilize soldiers over 100,000 in the 16th century were Ming Dynasty in China, India and Turks. Even the Hapsburg in Spanish Empire could only mobilize 50,000 soldiers in number. However, in Japan, at the end of Warring State Period in the 16th century, each feudal lord had about 20,000 soldiers at their disposal and in the Battle of Sekigahara (the battle which ended the Warring State Period and opened Edo period) in 1600, over 150,000 soldiers were mobilized. The number of soldiers employed in this battle exceeded the number of soldiers in all Europe combined. From this, it also followed that the demand for swords, guns as well as food was accelerated, and those militia groups who could not keep up with the provisions were naturally at disadvantage. Driven by such necessities, Japanese were forced to find a way to maximize the communication system amongst themselves, while minimizing the leak of information to the outsiders. One of my aims in this paper is to briefly explain in what aspects of warfare Ninja were employed for and to what extent. In passing, a general philosophy of Ninja is discussed. Then, I will present their methods of activity as the collectivity in Japanese empirical science, discuss about the medical practices of Ninja, analyzing if what they practiced can be called a medical knowledge, and conclude with overview of their science. However, we must begin by discussing a little about the primary sources for my findings as well as a brief history regarding the origin of Ninja as such.
There are three Ninja manuscripts that we use as semi-primary sources. These are 1) Bansenshukai, 2) Shouninki and 3) Ninpiden. Each of these includes detailed descriptions on how-to guide for any aspiring Ninja to become one. Here, we might wonder why they cared to write down any of their secrets, if indeed their prosperity was dependent upon appropriation of their own unique knowledge. It seems to be the case that after entering Edo period, centuries-long conflict had ended, and there was not much demand for a covert political organization. Although Ninja still lived among people and trained for a possible employment in the near future, it became increasingly evident that Ninja were no longer needed in this peaceful time. As the name suggests, Ninja is someone who sneaks in with a sword over his heart. It signifies someone whose life is always in danger of death, requiring an attentive performance to do the job, but the current state of affairs offered them no such situations. Ninja families, hence, decreased in number and gradually less and less people practiced Ninjutsu. Fearing that the techniques that once determined the course of Japanese history would become forever lost, the offspring of Ninja decided to write down their secrets. In this way, they chose to preserve their tradition rather than disappearing into the shadow of the history. The nature of their writings is such that there necessarily includes fabrication of techniques later added to exaggerate their merit and incredible accounts of their activities. It is even plausible that, although Ninja who wrote down their tradition were admired as Jo-Nin, they did not quite understand some of the techniques described there. Hence, historians of science must not take their records at face value, and instead try to distinguish a fable from a true claim by comparing various manuscripts passed down to us as well as performing empirical experiments.
Their essential activities, however, are known to us beyond dispute. These can be reduced into four divisions mentioned above, namely 1) espionage, 2) counter-intelligence, 3) conspiracy and 4) unconventional warfare. Ninja were most fitted for all of these activities than any other groups of people. For instance, their ability to move swiftly in small numbers enabled them to deliver information about the enemy’s movement or act in disguise in enemy’s territories to collect inside information as well as leak false information in their favour. The performance of such activities depended on the knowledge of orology. Engaging in unconventional warfare too was a tactic often employed by Ninja. Ninja may throw poison into a well, where they know their enemy gets their water from, or set fire on their provisions. Disguising as a mercenary in the enemy’s territory and spreading a rumour, saying, “we have no chance of winning this war, it’s all over!” also served as a way to demotivate the soldiers. 
III: Philosophy of Ninja
Seeing that Ninja primarily served as a covert military organization, who would basically do whatever is asked to do, there is always a concern for the potential employers whether Ninja would not betray them in the end. What distinguished Ninja from bandits or simple criminal organizations were the codes of conduct they firmly adhered to. These are the principles of Jo-nin. Jo-nin means a ninja par excellence. Jo-nin must be “dutiful (giri) to the employers, without desire… knowledgeable in all matters… faithful to Confucianism and Buddhist teachings… respectful to codes of warriors (i.e. Bushi-do)… familiar with the geography and customs… skillful at tactical thinking” and so on. In reality, no one is this talented, and most Ninja were not erudite. These “codes” of conduct are therefore to be taken only as an ideal to be aimed at, and not actually laws that were enforced upon Ninja. This “principle as an ideal” seems to be the central theme in Ninja philosophy, as we will see over and over again in their science. As a matter of fact, Ninja were internally bounded between their members more than they were to their employers, and they believed it utmost duty to help one another in times of difficulties. This gave them a limited but relative freedom as to exchange information amongst them, and Shouninki tells us that this is why some first rate secrets were often concealed from them in their employment.
Ninja philosophy is essentially derivable from Chinese philosophy, though some are more obvious than others. Shouninki, for instance, mentions Sun-Tzu’s The Art of War as the central source for their philosophy, according to which Ninja are supposed to be able to “master the languages of their enemy and become one of them.” Further, they must “talk about an event as if you were there when you were not, speak of someone whom you have never met as if your best friend, buy things when you do not have any money and get drunk when you have not drunk at all. You go out at night and never stay at an inn, sometimes you get surprised by the voice of a deer, and hide in the shadows of the trees, avoiding moonlight and ensconce yourself in an uncomfortable place.” It ends the chapter with a remark “what a sad life and full of hardships we suffer! Yet, one must keep such experiences to yourself, lest not to divulge his identity to others. It is probably for these reasons that people think Ninja are strange. But let them think so, if they do not understand the hardships we go through. If we are asked, ‘Aren’t you stupid to live like that?’ let us respond, ‘yes, we are.’ This too is a way of Shinobi. The Unreal is the Falsity and the Real is the Truth.”
Here, the concept of Falsity-Truth dichotomy deserves some attention. In the philosophy of Ninja, everything is flowing and nothing is absolute. This is an obvious reference to Yin-Yang theory, which states everything is in motion. The success in a mission depends upon perceiving the change in things accurately, and knowing when to act. Shinobi states that whatever disadvantageous is the Falsity and whatever causes benefit and power to them is the Truth. Hence, when times are in the Falsity, one must wait until the right time comes. Acting hastily could ruin the entire operation as well as his own life. “When samurai warriors fight one another, it is a fight between the Truth and the Truth, but when a Ninja fights, you are always the Truth and the enemy always the False. In such cases, it is important to have the enemy believe he is winning until his ultimate defeat.” It is by this means that Ninja can make himself the Truth while keeping the enemy the Falsity. This philosophy is vividly seen in the chapter that talks about Enemy Prevention. Here the text says that “even if the opponent becomes angry obviously because you did something wrong, you should blame the responsibility to someone else and disappear as soon as possible. They say that blaming others for what you did is wrong, but that is just what they say in general. There is no need for you to think such a general principle also applies to you.” Although I have said that Ninja help one another in times of difficulties, we must not forget that what is of utmost importance to them is the fulfillment of their mission. It would negate their whole purpose of existence if helping out a fellow Ninja would mean abandoning the mission. In order to complete the mission, Ninja must do whatever it takes. As such fidelity to mission takes precedence to ethics, they can justify aforementioned unconventional warfare, such as poisoning the enemy, destroying their weapons in advance as well as assassination and arson. Following this philosophy, Ninja believed anything is accomplishable. If you still fail at your mission, then your reason may be troubled by emotions or preconceptions about things, which lead to a lack of conviction or impatience. To prevent this psychological instability, one must maintain the qi at all times. As with Taoism, Ninja teaches us that “human mind is mysterious and comprehends natural elements (activities) namely Wood, Fire, Earth, Metal and Water. Human heart is thus like the universe itself… When our heart is at calm and observes nature, we can adapt to any situations like flowing Water. It is like Fire changing its force depending on what it burns, and Wood growing roots and leaves in their surrounding climate. If wind tries to sweep down the Wood, accept it and you will wave in unison with the wind, but should you resist it, you will be defeated. Similarly, Metal is hard but changes its shape according to the human deeds. Earth relates to all the Five Activities, and keeps nature in harmony.”
In this way, Ninja must obey the natural occurring of events as they unfold, and act without going against the flow of the nature. Only by attaining this state of detachment of the self can we recognize pure reason, and only through the use of pure reason can we be mindful to the subtleties in nature. Thus, Ninja attack with the absolute detachment of the self that “even if you ask for him, he is shapelessly merged into the universe, and even if you ask of him, he is so detached that there is no answer. He will act only in accordance with the pure reason.” This part of Ninja philosophy is largely taken by Zen philosophy, which states that “the state of bodhisattva is inexpressible in words and is only attainable by pure experience.”
This view of activities as a part of the cosmic nature and that we must respect the laws of nature is prevalent in Chinese medical philosophy.
IV: Philosophy of Medicine
It is unfortunate that Ninja did not elaborate on their medical theories and seemed to think it unimportant to subject their views to a theoretical analysis. This makes sense to some extent, since Ninja were not philosophers or scientists by profession, but only agents in covert operation. They had neither time nor interests in knowing how things worked, but only in that they worked. As a result, the parts in Shouninki where medicine is treated are limited to describing how to make what and when to use it. In fact, there are only two places where medicine is explicitly mentioned in Shouninki, and only few places mention medical recipes in a more philosophical manuscript Bansenshukai. But we know that medical knowledge played an important role in Ninja activities. This is because one of the most basic tasks of Ninja was to get information in the local areas. Whenever people needed to hide their identities in a foreign village, they would often disguise either as monks or as pharmacists in those times. Ninja, too, followed the suit. But they only needed the medical knowledge to the extent necessary, i.e. it must have been enough to be able to answer questions about medicine they were selling when asked about them. Because they were not certified doctors or experienced in medical knowledge but rather “part-time doctors”, they carried simple medicine that would most often be used by the general public. This worked in their advantage, as these medicines would be useful for Ninja themselves during their missions.
Chinese medical philosophy is explained in complex combination of Yin Yang theory and the Five Agents doctrine. Yin and Yang are opposite but complimentary forces, elements or principles, the former being “negative, passive, weak, and destructive, and [the latter,] positive, active, strong, and constructive.” Yin Yang theory sees the events as containing both yin and yang qualities, thus “[e]ach thing or phenomenon could be itself and its contrary,” that is to say, “Yin contains the seed of Yang and vice versa, so that, contrary to Aristotelian logic, A can also be Non-A.” Adding to that is the Five Agents doctrine, which introduces to Yin Yang theory a concept of rotation. These Five Agents are conceived of at the level of Chinese cosmogony; for instance, Water generates Wood, which generates Fire, which generates Earth, which generates Metal, which then generates Water. The concept of generation here is similar to the Four Elements in Greek philosophy. In Chinese philosophy, however, Water assumes an important basis, and is the beginning of the sequence. There are two other sequences that are significant in understanding how the agents interact with one another. One is called the Controlling Sequence in which each Agent controls another and is controlled by one. Thus, Water controls Fire, Fire controls Metal, which then controls Wood, which again controls Earth and back to Water. This sequence ensures that the balance is maintained among the Five Agents. It self-regulates its balance as well, in combination with the generating sequence we have just mentioned. In this model, Water controls Fire, but Fire generates Earth, which controls Water. Similarly, on the one hand, Water controls Fire but also generates Wood, which controls Fire, and so on. This sequence is a key model for sustaining health when dealing with sickness or illness. The second sequence is called The Insulting Sequence, which works the opposite of the Controlling Sequence. Hence, Water insults Earth, Earth insults Wood, which insults Metal, which insults Fire, and which insults Water. When there is an imbalance in the body, causing sickness, it is normally because agents are insulted and the order no longer maintained.
In this way, Chinese model may be comparable to that of Greek humoural medicine. What is different from the Western medical thought is that each Agent has so many correspondences, and what makes it more complex is that it is interlaced with the Five Agents doctrine. Just to give an example, Water corresponds to the kidney in the organ; Wood corresponds to the liver; Fire to the heart; Earth to the spleen; and Metal to the lungs. In this scheme, the liver controls the spleen and generates the heart; the heart controls the lungs and generates spleen; the spleen controls the kidneys and generates the lungs; the lungs control the liver and generate kidneys; and the kidneys control the heart and generate the liver. Further, Yin and Yang correspond to body parts, such as Front-Back, Body-Head, Interior-Exterior, Structure of organs-Function of organs, and Yin organs-Yang organs. Just as the Five Agents aim at maintaining the balance in the body, Yin-Yang theory is applied to medicine to tonify either Yang or Ying quality or to eliminate the excess of Yang quality or eliminate the excess of Ying quality.
For example, a seed of a fruit such as of a peach corresponds to Metal in the Five Agents scheme, whereas in Yin-Yang theory, it corresponds to Yang quality since it is hard rather than soft. Metal also corresponds to repelling of evil spirits, generating Water, which is the beginning of the generation cycle, but it also has a quality to insult Wood, which represents Spring. In this way, Japanese people throw roasted soybeans outside the house on the first day of spring to repel evil spirits for the new cycle of the year. We also throw the beans inside the house, exclaiming “Demons out! Luck in!” The soybeans signifying Metal goes out to guard against the evil demons, while at the same time, the quality of Metal inside the house is weakened by us gnawing the beans to strengthen the quality of Wood.
As you can see, while such was the basic working principles in medical thought in the pre-Modern Japan, it would have been too much to ask of crypto part-time doctors to memorize and understand all the correlations among the Yin-Yang Five Agents Doctrine, not to mention the measurements and dosages of each medicine. In effect, even one of the most celebrated Buddhist doctors in Japan once lamented, “[o]ne cannot memorize all this.” Did Ninja then not understand medicine when they talked about painkillers or antidotes? Their poisons seem to have worked well enough and it is historically true that they made poisons out of surrounding plants to kill. In order to determine their philosophical framework regarding the formulation of medicines, we will now take a look at two of the medical formulae that appear in the most authoritative text and discussed in detail, and see if they follow the medical philosophy of the East.
The first medicine is called “Suikatsu-gan” (水渇丸), which, if taken in a dire need of quenching your thirst, will make you suffer no more. The recipe for which is rather simple and composed of three ingredients:
− 梅干しの肉 １両（４匁） ＝ Plum without skin, 37.3
− 氷砂糖 ２匁 ＝ Rock sugar, 7.5g
− 麦芽（麦門冬／麦角）１匁 ＝ Malt, 3.75g
You will mix them and powder it, and then make a small ball and take one whenever you are thirsty.
The second medicine is “Kikatsu-gan” (飢渇丸), which will render no food necessary when taken three each day.
− 人参（キタネニンジン) １０両 ＝ carrot, 373g
− 蕎麦粉 ２０両 ＝ soba flour, 746g
− 小麦粉 ２０両 ＝ (all-purpose) flour, 746g
− ヤマノイモ（山芋 ２０両 ＝ Japanese mountain yam, 746g
− 天草／耳草（はこべ類） １両 ＝ stellaria (or, chickweed, stitchwort) 37.3g
− ヨクイニン（ハトムギ果）１０両 ＝ Job’s Tears, 373g
− 糯米粉 ２０両 ＝ rice flour, 746g
When you mix them all, you marinate it in five liters of sake for three years, and when sake completely dries out, you make balls of about two centimeters in diameter.
Now, if you follow this recipe verbatim, it will rot before even sake gets dried out. Perhaps you will need to find a way to dry sake out much quicker. But even if it worked, will it have the effect they claim it does? Will it work in theory? As I have mentioned earlier, Ninja did not elaborate at all on their theories but seemed to care only that they worked. Any reflections on their medicine in light of Yin-Yang Five Agents doctrine will be necessarily speculative and unfounded. So the question, which concerns us most, now becomes: did they work? The answer seems to be a disappointing one, that is, yes and no. Although Ninja medicine was not compatible to the Yin-Yang Five Agents doctrine, it did accord with the other prevalent philosophy in medicine as well as in Buddhist philosophy, that is, the transformation or the imbalance of Qi. Qi is the source of all movement in human physiology, and assumes “different forms depending on its state of condensation or dispersal… [and] is transformed, changed, transported, it enters, exits, rises, descends and disperses.” Further, Qi forms a material body and has a Yin quality when condensed, while it has a Yang quality when it is dispersed and in motion. So when “Qi is flourishing there is health, if it is weak there is disease, if it is balanced there is quiet, if it moves in the wrong direction there is disease.” Indeed, already by the 13th century, it was commonly believed that the disorder of Qi within the body was the cause of many illnesses, as a famous Buddhist monk in the 13th century repeatedly remarked in his medical corpus that “medicines that readjust the qi thereby treat the myriad illnesses and can’t go wrong,” categorizing the illnesses arising from the mind as internal causes of ailments. After stating that “internal causes [of ailment] are found where illnesses are produced from the disorder of the seven qi vital energies of joy, anger, melancholy, worry, sadness, fear and fright,” he recounts us “the well-known Chinese story of Yueguang, who exhibited severe symptoms and became ill because he had imagined that he had swallowed a snake.” Apparently, he had mistaken the reflection of a bow in his drink for a snake and believed that he had drunk the snake. In this way, it was conceived that the evil passions and the source of mistakes arise from the imbalance of the qi, which is in the mind. Once this becomes apparent, “emotions are neither produced nor activated, the blood and qi are harmonized, and the mind and the body are at rest.” This philosophy resembles much of what we have spoken of when we saw that Ninja abided themselves by the principle of absolute detachment as discussed towards the end of Shouninki. I would also like to mention in passing that when Ninja talked about using amulets to protect themselves from danger and spells to accomplish their missions, they also stated explicitly that “to actually expect they will work is to be utterly ignorant about the world, yet because there is no particular reason to disregard them as mere superstitions, the decision to use them or not depends upon you.” As the commentator of Shouninki also explains, it seems that if something works actually or not was not a concern to them, as long as it works at least apparently. It is in this sense that Ninja medicine worked. That is, if amulets or medicines had a positive influence to control qi in the person, that is all what was required for. From these considerations, it seems reasonable to suggest that the fruit of Ninja medicine lies in the maintenance of the order of qi rather than depending on the effects of the specific medicaments.
Now, what about poisons? Poisons enjoyed a distinct status in Ninja science in that they actually worked. However, here too, it seems that how they concocted deadly poisons had not much to do with the monastic medical philosophy, but simply to do with pure experience. With the healing medicines, it was possible to argue that whether they work or not depended on how detached you are according to the Buddhist philosophy, but because one cannot deceive another with a poison that does not actually work or whose effects depend upon the enemy’s conviction that it works, Ninja had to actually test them in a number of occasions, and they left recipes for those poisons that did work. In fact, although there are only so few references as to healing medicines, the list of poisonous medicines is incomparably large. There are recipes that use the ingredients like toad’s oil, red spider lily, Japanese star anise, as well as cannabis and nux vomica, but here I will list a few of the deadliest poisons. The first of which is the poison called Machin (馬銭), whose main ingredient is nux vomica. It is a deciduous tree native to India and Southeast Asia, which is highly toxic. Strychnine in the seed is said to kill you if you ingest a mere gram. Ninja used this primarily to kill dogs that guarded the house they tried to sneak into. They studied psychology and human behaviors, and they were able quite accurately to predict how people would react to unknown noises at night or if they would fall for a deception. But dogs were different. Even Ninja could not read dogs’ mind to deceive, since dogs react to movements immediately. In a way, dogs were the Ninja’s enemy par excellence, whom they could not draw in to their side. So the elimination of dogs would give them an advantage, and hence was the basic Ninjutsu. They did this by first taming the dogs of the house, giving them rice balls, a few days before they would sneak into. If they were completely tamed by the night of mission, they would cause them no harm, but if the dogs were still cautious of them, Ninja would give them rice balls mixed with powdered nux vomica. The dogs would remain at the verge of death for a while, and if the missions were accomplished before they died, Ninja would give them water, which would revive them. If you absolutely had to kill the dogs, you would give them rice balls mixed with nux vomica and finely ground iron. Another famous poison is called, loosely translated, ‘the poison of the teahouse.’ It uses Gyokuro, a rare type of tealeaves. You make an extra strong tea, and pour it into a bamboo tube. You will then bury it under the earth, and take it out after a month. It is said that a few drops would make you sick in one month and die in two months. Yet another unique poison includes a mole, a newt and the blood of a snake mixed with other herbs. If you light fire on it and throw it into the enemy’s house, the enemy who inhaled the smoke would sleep or hallucinate, and would die in 70 days. But this probably did not work. There is another medicine that probably did not work but is praised as a ‘medicine of the enlightened,’ which uses black soybeans and cannabis in the ratio of 5:3. You mix them, powder it and make a ball out of it. Then, you smoke it, and powder it again. If you drink a cup of tea made from this formula, you can apparently maintain your energy and do not have to eat or drink for seven days. If you drink two cups, you can stay alive without food or drink for 49 days, and with five cups of it, you will live for 16,807 days.
As we can see, Ninja corpus includes medicines that would and would not work in tandem. This is probably due to the fact that they wrote the manuscripts long after their prime time, and they were writing in part to heroicize their past glory in times of relative peace, but also because their focus in medicine was more about controlling the qi rather than following a theoretical formula.
V: Psychology, Animal Philosophy and Observational Science
Although Ninja medicine lacked in a theoretical rigor, their observation excelled in natural philosophy. Some of their astonishing scientific discoveries based on pure observation include the colour of their costume, cat clock, as well as the accurate observation of the sleeping patterns of human beings. People now take it for granted that Ninja wore a black uniform, but a Ninja scholar, Yamakita, rightfully points out two major problems with this common belief. First of all, wearing such a costume all the time is utterly conspicuous, and illustrates insanity. Why would you wear clothes that speak out ‘I am a Ninja!’ when you want to be invisible to the public eyes? As has been stated earlier, the activities of Ninja primarily include information gathering, as such, they travel between the cities and meet people very frequently. If they really wore black uniforms at night, they must have carried additional clothing for the daytime activities. But carrying extra stuff in addition to several items to be used in their activities is already a huge burden. This is also why Ninja had to invent medicines such as Suikatsu-gan (mentioned above) that are tiny and do not take up space. Considering practicality, it would be much easier and convenient for them to have worn clothes that are darker in colour but could be worn in daytime as well without appearing suspicious. Secondly, pure black colour is rather expensive. Ninja who would sneak into houses and castles were normally the lower rank Ninja, and it would not have been plausible that they could have afforded to have uniforms with pure black colour made for them for everyday use. These are rather technical difficulties, but there is one more crucial scientific reason why they could not have worn black uniform. We now need to look at the structure of human eyes briefly. In retina of the eye, there are two layers of segments: retinal cone photoreceptor cells and rod cell outer segments. The cone photoreceptor cells are converged in the center of the eye and detect colours in daytime or in bright spaces, whereas rod cell segments have photic sense and can see in dark places but detects only light or dark. In short, human eyes can detect colours well only when we see something with the center of the eye in fairly bright places. On the contrary, we cannot detect colours well if we see something at the edge of the eye or in dark places. The cone photoreceptor cells stop detecting colours starting with the ones with longer wavelength. So the colour red which has the longest wavelength in the primary colours become invisible first, and then yellow loses its colour, leaving the blue alone to be seen in the end. This is why at dawn, everything looks somewhat blue-ish, because the photoreceptor cells are inactive due to the lack of light, and as a result, we see things with the rod cell segments of the eye. When you consider this fact, pure black clothing is actually a disadvantage. The reasons being that, even at night, it is not a pitch black. There are moonlight, starlight and candlelight from houses, and almost everything including plants and rooftops reflect tiny amount of light. However, pure black suit would not have any luminosity, so in a place where your surroundings emit some amount of light, you would be more easily found because of the contrast your clothes creates. Now, in daytime, if you hide behind trees or walls, it can be quite dark. Of course, if somebody sees you directly from the front, then there is no chance you can avoid detection, but wearing darker coloured clothing that has similar degree of luminosity as the surroundings would be beneficial if you are seen with the corner of the eye, you may be able to escape their detection. Now we see that clearly, black coloured uniforms are not practical for a covert operation. In fact, the reason why we see Ninja wearing pure black in movies and shows is because it is cool and also it is easy to spot. What Ninja in fact wore was a uniform in dark reddish brown. This is because the cone photoreceptor cells lose colour red before they lose yellow and blue, it is the first colour to disappear in the dark, and because it is not completely pure black, it has the benefit of attaining some level of luminosity, allowing it to be mingled with the surroundings.
That Ninja were particularly observant as regards with lightness and darkness is also seen from their use of cats in determining what time it is. Clocks existed back then, but they were not portable. In order to find roughly what time it is, Ninja looked at eyes of the cats nearby, and made a song to memorize: 6 when it’s round, 5 or 7 when it’s egg-shaped, 4 or 8 when it’s a seed of a persimmon, and 9 when it’s a needle. This finding of knowing the time by looking at the cat’s eyes was employed to argue in support of knowing the best time to sneak into a house. Because the success of sneaking in depends on the time when they sneak in, they left an instruction on when to do it with most success in Shouninki. Here I will quote the article almost in full citation.
“although there is not a fixed time that is best to sneak in, you should in general aim for when people are off-guard, like when they are busy. Do not be inpatient. Some of the best times are at twilight, 10 pm, midnight, 4 am, 6 am, noon and 6 pm. Should you not know what time it is, you should make an effort to know it through experience. Even cat’s eyes become thin or round depending on the time of the day, it is implausible that humans cannot be susceptible to it. In general, people go to bed at around 9 to 10, and are in a deep sleep at 1 to 2 am, waking up at 5 to 6 am. If you are in a deep sleep, the breathing sounds are irregular, if they are regular, you are probably pretending to be asleep. Even if they are deep asleep, people often wake up regularly and you need to be careful to detect when they are in a deep sleep and when they are in a shallow sleep… When climbing the rooftop and walking on it, you will use your sword as a stepping stone and try not to make any sound. If you see something unfamiliar on your way, throw a stone at it and see how it will react. If they notice your presence, use the technique called “Ryofuri (両降り)”, or “Falling from Both Sides”, and throw a stone at the opposite direction to the one you are going to, and make a run for it.”
Ninja were well aware that the eyes of cats are much more receptive and their eyes could be thin even at noon if they are in the shadow. The commentator of Shouninki states that the intent of this article is to encourage you to use all the senses possible, and detect any information necessary to know the time by listening for bells at temples, cries of animals or smells of food being prepared, etc… What is most interesting here is the depiction of sleeping patterns. This is a clear reference to what we now call REM sleep, when breathings become irregular and when we most likely dream. It appears once every two hours or so, and lasts for 5 to 30 minutes. People in REM sleep look as if they are at the verge of waking up but in fact they are in the deepest sleep and would not easily wake up.
Ninja also employed psychological means to hide their identity, and they learned their survival techniques mostly from animals around them. For instance, when you hide by the trees or in the shadows, it was possible for people to still hear heartbeats or breathing sounds especially at a quiet night. There was not a single noise of a car to help you make up for any sound you make. In such situations, Ninja often imitated animals for a disguise. Although this sort of techniques is often used for a comical effect nowadays, when your life depends on how well you imitate, the imitation must have been of the highest quality. When people hear some unknown noise, they would be uneasy and a desire to know what that noise is arises. Ninja observed this fact, and they answered that if people are anxious to know what the noise they had just heard is, only false information was necessary to make them believe they understood the cause of it. If they heard a cat immediately after hearing the strange noise, they would reason that the sound they just heard must have been of the cat they are hearing now. Once they are convinced, even if they hear a stranger sound consequently, they would assume that the cat must be of a big one or must be sick, and so on, to try to stick to the original conclusion they have reached. Further, because they imitate the sound of animals, even if they constantly make some noise, people would just think that cats are being noisy. Since cats and dogs were often found in and out of the house, Ninja were particularly trained to disguise as cats or dogs and were good at it. In addition to this, Ninja used a number of hiding techniques learned from various animals. The famous instances of which are ‘Kitune-gakure (狐隠れ)’ [the Way of Foxes], ‘Tanuki-gakure (狸隠れ)’ [the Way of Raccoon-dogs], and ‘Kakure-mino (隠れ蓑)’ [Hiding in the Straw]. Kitsune-gakure is the one in which Ninja hide underwater. It received its name from the fact that foxes have strong smell, so when they are chased after by hunting dogs, they are said to escape their detection by traversing in the water. Here, Ninja will usually leave the face above the surface to breathe. With water grass and algae around, it is difficult to be found. They probably did use a bamboo tube or a scabbard (a sheath; a case for katana) to breathe, completely submerging themselves under water, but the problem with it is that it would be unnatural and overtly conspicuous if there is a bamboo tube sticking out where there is nothing else around. In any case, to prevent Ninja from hiding, samurai warriors often cleared any water grass or algae in the ponds in advance. This is also known as ‘Suiton no Jutsu (水遁の術)’ which is a collective name for a escaping method using water. Although it is famously employed often in Ninja themed stories, it is not something you would want to do, as you would be completely wet and anyone who sees you would be suspicious of you. On the contrary, Tanuki-gakure is when you hide on the trees, camouflaging with the leaves. As long as you are not seen climbing the tree, it probably works quite well. We are not used to checking what is above us, so this too is a tactic that studied human psychology. Lastly, Kakure-mino is what you use when there is absolutely no time to climb trees or hide under water without making any splashing sound. In Kakure-mino, you will find a space or a container of rice, for example, to hide and wait until the enemy is gone. If you do this, however, you are deprived of your freedom to move, and once you are found, you probably get killed without a further ado.
There are many other tricks or items Ninja used in their daily operations, some of them ingenious while others are utterly useless. Before I conclude my paper, I would like to mention one more invention of Ninja that most likely did not work but is seen as the most characteristic item of Ninja, and that is: Mizugumo. As you can see in the photo, it is virtually impossible to walk on water, let alone run from the enemy. It is apparently made with a foxglove tree, but in order to float a person weighing 50 kg, you will need to wear a cone tube that is 50 cm in diameter and 20 cm in height on both legs. You may be able to float, but you certainly cannot move smoothly. Further, you would be looking like a crab, unable to move on water, that you are an excellent target for the enemy to throw stones at. Although it is hoisted as one of Ninja’s most famous invention, and also is recorded in Bansenshukai, it is generally attested that such a device did not exist but was later added by the offspring who left the manuscripts.
This concludes my findings on the philosophy of Ninja. I have illustrated that Ninja were a professional spy organization, and hence needed to master techniques to find out inside information, poison the enemy before the battle begins, and disguise themselves so as to escape the enemy’s detection. In doing so, I have briefly discussed the basic Chinese philosophy that was on the foundation of Japanese medical theory since the 12th century onwards, such as Yin-Yang theory, Five Agents doctrine as well as transformation of qi energy, and I have argued that Ninja medicine had not much to do with any particular philosophical doctrines but rather much to do with being in control of their own emotions and qi energy. In this respect, it is largely a reflection of Zen philosophy, more so than of other schools of thought. I then concluded with some discussions on their psychology, animal philosophy as well as scientific discoveries for your information, which I term observational science. In sum, I believe Ninja did not know and did not have any interest to Chinese medical corpus but rather collected information on their own through experience and observation. Their interest rather lied with the philosophy of warfare, and although they obtained a lot of information from the Chinese books, their scientific enquiry remained limited and never left their own circle for criticisms and improvement.
 This would be the late 16th century, as is discussed below.
 Atsushi Yamakita, All Things About Ninja (概説 忍者・忍術) (Japan: Shinkigensha, 2004), 21. Also as opposed to the number of soldiers carrying guns being only 10% of the army in Europe, 30% of the army were equipped with guns in Japan.
萬川集海 was written in 1676, written by Fujibayashi Yasutake.
正忍記 was written in 1681, written by Fujibayashi Masatake.
忍秘伝 was written in 1652-1727, presumably by Hattori Hanzo in 1560, originally.
 That is, to be in the state of absolute detachment from the self, as if to kill his own heart. See Shouninki, 39.
 Yamakita, All Things About Ninja, 27.
 See Wing-Tsit Chan, A Source Book in Chinese Philosophy, 246. “the yin ang theory has also put Chinese ethical and social teachings on a cosmological basis. It has helped to develop the view that things are related and that reality is a process of constant transformation.
 Satori (悟り), or enlightenment.
 Also known as ‘Five Movements’ or ‘Five Actualities’.
 Wing-Tsit Chan, A Source Book in Chinese Philosophy, 244.
 Giovanni Maciocia, The Foundations of Chinese Medicine: A Comprehensive Text for Acupuncturists and Herbalists, 1.
 The sample list of correspondences of the Five Agents can be found in the handout. (from p.21)
 Makoto Takemitsu, The Wisdom of Onmyo-do Every Japanese Should Know, 126-127. The festivity is called Setsubun for those who wish to further research on this topic.
 Andrew Edmund Goble, Confluences of Medicine in Medieval Japan, 51. Kajiwara Shozen (1265-1337)
 Referenced in Shouninki, 52.
 Yamakita, All Things about Ninja, 183.
 Maciocia, The Foundations of Chinese Medicine, 59.
 Goble, Confluences of Medicine, 63.
 See Gokuhi-den in Shouninki, 189.
 This view is also contested by the other scholars working on Ninja, such as Yamakita Atsushi. See his All Things About Ninja.
 Ibid., 96-97. See also Masayuki Yamaguchi, Shinobi and Ninjutsu, 155.
 Yamakita, All Things About Ninja, 183. However, I have yet to locate where he is getting this information, as Gyokuro was first used in 1835, a long after Ninja had dwindled.
 Ibid., 202-203. In Bansenshukai, it is stated that “6 when it’s round, 5 or 8 when it’s egg-shaped, 4 or 7 when it’s a seed of persimmon and 9 when it’s a needle.” Yamaguchi Masayuki thinks it is a mis-scribe and offers the above mentioned interpretation.
 Shouninki, 90-92. Here, I quoted the article almost in full.
 Yamakita, All Things About Ninja, 207-208.
 Literally, ‘water-spider’.
 Yamakita, All Things About Ninja, 177.
 It was most likely taken from a Chinese book on warfare that I have not yet been able to locate.