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