In the second case, the external ideas that have entered the brain are disintegrated there and unite their atoms with the other atomic knowledge already within us. An idea is digested, assimilated. Assimilated, it then becomes very different from what it was when it entered the intelligence. Like intestinal absorption, mental absorption is, therefore, an excellent though indirect way of acquiring knowledge. In both cases, the ideas, like the aliments, will be known, not immediately, but by their effects. Thus men know hereditarily that certain ideas are individual or social poisons, and that others are equally favourable to the welfare of the individual or to the development of a people. But, in this order, notions of utility and of harmfulness are much less precise. We have seen a certain idea, reputed to be very dangerous, contributing to the health of a man, of a family, of a society, of civilization itself. Ideas are extremely workable, plastic. They take the shape of the brain. There are perhaps no ideas that are bad for a healthy brain whose form is normal. There are perhaps no good ones for a brain that is sick and warped. III But let us come back to our tree or our ox. This ox can enter us in one of two ways. First, partly, but really, in the form of food. What we absorb of it in that way cannot, evidently, be known as ox. It reaches our knowledge only through its effects--strength, health, gaiety, activity, depression. Even were this absorption total in the case of a small animal, digestible in all its parts, the result, from the point of view of immediate knowledge, would be the same, since the object becomes resolved into elements which render its form unknowable. The other manner--that which brings into play the external senses--will make us know the ox, in appearance as such, in reality as image of an ox. What is the true value of this knowledge? We must here return to this question, in order to enter more easily upon the second part of this essay. Truth has been very seriously defined as conformity of the representation of an object with this object itself. But that solves nothing. What is the object itself, since we can know it only as representation? It is useless to carry the discussion further. We shall turn indefinitely around the fortress of idealism, without ever finding an opening, or any weak point. We shall enter it never, no argument serving as a bomb against its solid walls.