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.
Man has debated at length and will continue to debate at length--the world having been assigned as a theatre for his debates--concerning the origin of knowledge; but, apart from the question as to what the real truth about this origin may be, which we will leave until later, it is a certainly ascertained fact that in the apparential order of things, in the life of beings who are endowed with a certain more or less cloudy faculty of knowing and perceiving, or who at any rate appear to act as if they were so endowed, knowledge is exhibited to us as bound up with the necessity of living and of procuring the wherewithal to maintain life. It is a consequence of that very essence of being, which according to Spinoza consists in the effort to persist indefinitely in its own being. Speaking in terms in which concreteness verges upon grossness, it may be said that the brain, in so far as its function is concerned, depends upon the stomach. In beings which rank in the lowest scale of life, those actions which present the characteristics of will, those which appear to be connected with a more or less clear consciousness, are actions designed to procure nourishment for the being performing them.