This positive transformation of the doctrine of final causes had already been clearly sketched by the philosophers of the XVIII. century whom Comte knew very well, by Diderot, by Hume, by d’Holbach. Hume says, for instance,[66] “It is useless to insist upon the uses of parts in animals or in plants, and on their curious adaptation one to another. I should much like to know how an animal could subsist without this adaptation. Do we not see that if it ceases he perishes at once, and that the matter of which he was composed takes some other shape?” And d’Holbach, “These wholes would not exist in the form which they bear, if their parts ceased to act as they do; that is to say, ceased to be arranged in such a way as to lend themselves to being mutually helpful to each other. To be surprised that the heart, the brain, the eyes, the arteries, etc., of an animal act as they do; or that a tree produces fruit, is to be surprised that a tree or an animal exists. These beings would not exist or would no longer be what they are, if they ceased to act as they do: this is what happens when they die.”[67] Comte makes this criticism of the doctrine of final causes his own. But, faithful to his maxim, “We only destroy what we replace,” he claims to substitute a positive principle to this metaphysical doctrine, which preserves the elements in it which are compatible with the scientific method. It is the principle of the conditions of existence. In virtue of this principle, by the very fact that such an organ is part of such a living being, it necessarily co-operates in a determined although perhaps unknown manner, with the totality of the acts which make up its existence: an organ no more exists without a function than a function without an organ. But it in no way follows from this that all the organic functions are performed as perfectly as we could imagine them to be. For instance pathological analysis demonstrates that the disturbing action of each organ upon the whole of the economy is very far from being always compensated for by its utility in the normal state. “If, within certain limits, everything is necessarily arranged in such a way as to be able to exist, we should seek in vain, in the majority of effective arrangements, for proofs of a wisdom superior or even equal to human wisdom.”[68]