The gorilla, the chimpanzee, the orang-outang, and their kind, must look upon man as a feeble and infirm animal, whose strange custom it is to store up his dead. Wherefore? And this primary disease and all subsequent diseases--are they not perhaps the capital element of progress? Arthritis, for example, infects the blood and introduces into it scoriæ, a kind of refuse, of an imperfect organic combustion; but may not this very impurity happen to make the blood more stimulative? May not this impure blood promote a more active cerebration precisely because it is impure? Water that is chemically pure is undrinkable. And may not also blood that is physiologically pure be unfit for the brain of the vertical mammal that has to live by thought? The history of medicine, moreover, teaches us that progress consists not so much in expelling the germs of disease, or rather diseases themselves, as in accommodating them to our organism and so perhaps enriching it, in dissolving them in our blood. What but this is the meaning of vaccination and all the serums, and immunity from infection through lapse of time? If this notion of absolute health were not an abstract category, something which does not strictly exist, we might say that a perfectly healthy man would be no longer a man, but an irrational animal. Irrational, because of the lack of some disease to set a spark to his reason. And this disease which gives us the appetite of knowing for the sole pleasure of knowing, for the delight of tasting of the fruit of the tree of the knowledge of good and evil, is a real disease and a tragic one. _Pantes anthrôpoi ton eidenai oregontai phusei_, "all men naturally desire to know." Thus Aristotle begins his Metaphysic, and it has been repeated a thousand times since then that curiosity or the desire to know, which according to Genesis led our first mother to sin, is the origin of knowledge. But it is necessary to distinguish here between the desire or appetite for knowing, apparently and at first sight for the love of knowledge itself, between the eagerness to taste of the fruit of the tree of knowledge, and the necessity of knowing for the sake of living. The latter, which gives us direct and immediate knowledge, and which in a certain sense might be called, if it does not seem too paradoxical, unconscious knowledge, is common both to men and animals, while that which distinguishes us from them is reflective knowledge, the knowing that we know.