But is this really a dead weight that impedes the progress of science, or is it not rather its innermost redeeming essence?  It is in fact the latter, and it is a gross stupidity to presume to rebel against the very condition of life.

Knowledge is employed in the service of the necessity of life and primarily in the service of the instinct of personal preservation.  This necessity and this instinct have created in man the organs of knowledge and given them such capacity as they possess.  Man sees, hears, touches, tastes, and smells that which it is necessary for him to see, hear, touch, taste, and smell in order to preserve his life.  The decay or the loss of any of these senses increases the risks with which his life is environed, and if it increases them less in the state of society in which we are actually living, the reason is that some see, hear, touch, and smell for others.  A blind man, by himself and without a guide, could not live long.  Society is an additional sense; it is the true common sense.

Man, then, in his quality of an isolated individual, only sees, hears, touches, tastes, and smells in so far as is necessary for living and self-preservation.  If he does not perceive colours below red or above violet, the reason perhaps is that the colours which he does perceive suffice for the purposes of self-preservation.  And the senses themselves are simplifying apparati which eliminate from objective reality everything that it is not necessary to know in order to utilize objects for the purpose of preserving life.  In complete darkness an animal, if it does not perish, ends by becoming blind.  Parasites which live in the intestines of other animals upon the nutritive juices which they find ready prepared for them by these animals, as they do not need either to see or hear, do in fact neither see nor hear; they simply adhere, a kind of receptive bag, to the being upon whom they live.  For these parasites the visible and audible world does not exist.  It is enough for them that the animals, in whose intestines they live, see and hear.

Knowledge, then, is primarily at the service of the instinct of self-preservation, which is indeed, as we have said with Spinoza, its very essence.  And thus it may be said that it is the instinct of self-preservation that makes perceptible for us the reality and truth of the world; for it is this instinct that cuts out and separates that which exists for us from the unfathomable and illimitable region of the possible.  In effect, that which has existence for us is precisely that which, in one way or another, we need to know in order to exist ourselves; objective existence, as we know it, is a dependence of our own personal existence.  And nobody can deny that there may not exist, and perhaps do exist, aspects of reality unknown to us, to-day at any rate, and perhaps unknowable, because they are in no way necessary to us for the preservation of our own actual existence.