Ninja, as almost everyone knows, are the legendary assassins, spies, and saboteurs of Medieval Japan. However, almost nothing that is said or claimed about ninja is actually true.
We can start with the word "ninja" itself. The word "ninja" is made up of two Chinese characters and literally means "stealthy one" or "one who endures." The first character, pronounced shinobi when it occurs in isolation, originally meant "to endure" in Chinese, but later acquired a secondary meaning of "stealth" in Japanese, while the second character, pronounced mono when it occurs by itself, is an extremely common character meaning "person," "thing," or "one". But the odd thing about the word "ninja" is that it doesn't really appear in Japanese written documents of any kind until the 20th century. Before the early 1900s, if there were any such thing as ninja, they must have had some other name.
Next we can move on to the so-called martial art of "ninjutsu" which was supposedly practiced by the ninja in olden times. While there is certainly a collection of martial arts techniques today known as "ninjutsu," as expounded by well known Japanese martial artist Hatsumi Masaaki, these techniques were not known as "ninjutsu" until they were given that name by Hatsumi's teacher Takamatsu Toshitsugu in the mid-20th century, long after the ninja had become a well-known figure in popular culture. So while Takamatsu's techniques were real martial arts with real historical lineages, they had not previously been associated with the ninja until Takamatsu cagily decided to capitalize on the popularity of ninja and call his techniques "ninjutsu."
Finally, we can look at historical records. Once again we find very little to support the legend of the ninja. The fact is that there are simply no historical documents attesting to the existence of the ninja in medieval times. The classic counterargument to this statement is of course that the ninja were SOOPER SEKRIT, so why would they leave any documents or evidence of their existence? Nevertheless, it seems somewhat unlikely that even a group of people as secretive as the ninja are supposed to have been could have left no trace whatever of their activities or even a single mention of their existence. Given this complete lack of evidence, the burden of proof would seem to fall on those who wish to claim that the ninja really existed rather than on those who suggest they might not have.
Defenders of the ninja are quick to point out that we know of several actual historical ninja. Three of the names most commonly mentioned are Hattori
Hanzo, Momochi Sandayu, and Ishikawa Goemon, but each of these examples fails to yield a verifiable ninja under closer scrutiny.
Hattori Hanzo, whose real name was Hattori Masanari, was real enough. A well-known retainer of
Tokugawa Ieyasu, his family had served the Tokugawa family
for several generations. But he was clearly a samurai, and was documented to have fought in over a dozen samurai battles before dying in 1596 at the age of 55.
As for Momochi Sandayu, it is impossible to say if he ever even existed, as his name does not appear in any records from the time was said to have lived according to later stories. While there was a Momochi family in Iga province during the Sengoku Period, they were simply a minor warrior family allied with a local war band, and there is no record of any members of that family possessing special martial arts techniques of any kind.
Ishikawa Goemon was an actual person who was caught sneaking into Osaka Castle by Toyotomi Hideyoshi, who punished him by boiling him in a giant kettle. But although later tales speculated that he must have been some sort of ninja, records indicate that he was executed for being a thief, rather than a spy.
So what can we really say about the ninja, in the final analysis? One thing we can say for sure is that there were certainly techniques and tactics which had been developed in Medieval Japan for infiltrating enemy territory. We can also say with confidence that there were assassinations, espionage, and sabotage. But what we lack any historical evidence of is special groups of people who practiced these techniques as a secretive martial art and hired themselves out to the highest bidder. It is clear that if we want to find ninja, we would need to conceive of "ninja" as a function rather than a special kind of warrior. If there were any ninja at all, they were samurai performing "ninja" type assignments (and even "samurai" was not an official social class during this period - it was merely an occupation).
But if all this is true, where did the whole idea of ninja come from in the first place? Well, what we do know is that in the peaceful Edo Period (1603-1868), long after the samurai had ceased to fight battles, there was an explosion of ninja-type characters in the thriving popular culture of the era. Traveling ninja shows, ninja houses, ninja novels, and ninja plays were all popular by the mid Edo period (although the term "ninja" was not used - rather, these characters were known as "oniwaban" - 御庭番 - or "shinobi" -志能備. Whether these tales were invented out of whole cloth or at least loosely based on some sort of actual secret tradition is very difficult to say, but it is clear that the image of the ninja as we know it in popular tradition today was created in the fertile minds of entrepreneuring Edo-period entertainers.