Plato, Timaeus

[81a] and of the whole body, each part drawing therefrom supplies of fluid and filling up the room of the evacuated matter. And the processes of filling and evacuating take place just as the motion of everything in the Universe takes place, namely, according to the law that every kindred substance moves towards its kind. For the bodies which surround us without are always dissolving us and sending off and distributing to each species of substance what is akin thereto; while the blood-particles, again, being minced up within us and surrounded by the structure of each creature as by a Heaven, are compelled to copy the motion of the whole; [81b] hence, when each of the particles that are divided up inside moves towards its kin, it fills up again the emptied place. And when what passes out is more than the inflow every creature decays, but when less, it increases. Now when the structure of the whole creature is new, inasmuch as the triangles which form its elements¹ are still fresh, and as it were straight from the stocks, it keeps them firmly interlocked one with another, and the whole mass of it is of a soft composition, [81c] seeing that it is newly produced from marrow and nourished on milk; and as the triangles contained therein, which have invaded it from without and go to form the meats and drinks, are older and weaker than its own, it divides and overcomes them with its own new triangles, and thus renders the creature large by feeding it on many similar substances. But when the root of the triangles² grows slack owing to their having fought many fights during long periods, [81d] they are no longer able to divide the entering triangles of the food and assimilate them to themselves, but are themselves easily divided by those which enter from without; and in this condition every animal is overpowered and decays; and this process is named “old age.” And finally, when the bonds of the triangles in the marrow which have been fitly framed together no longer resist the strain but fall asunder, they let slip in turn the bonds of the soul, and it, when thus naturally set loose, flies out gladly; [81e] for whereas every process which is contrary to nature is painful, that which takes place naturally is pleasurable. So too, in like manner, the death which occurs in consequence of disease or by wounds is painful and violent, but that which follows on old age and constitutes a natural end is the least grievous of deaths and is accompanied by more of pleasure than of pain.

The origin of disease is plain, of course, to everybody.