Hippocrates De prisca medicina 24

[24] And, as has been formerly stated, one ought to be acquainted with the powers of juices, and what action each of them has upon man, and their alliances towards one another. What I say is this: if a sweet juice change to another kind, not from any admixture, but because it has undergone a mutation within itself; what does it first become?—bitter? salt? austere? or acid? I think acid. And hence, an acid juice is the most improper of all things that can be administered in cases in which a sweet juice is the most proper. Thus, if one should succeed in his investigations of external things, he would be the better able always to select the best; for that is best which is farthest removed from that which is unwholesome.

[24] As I have said before, we must examine the powers of humours, and what the effect of each is upon man, and how they are related to one another. Let me give an example. If a humour that is sweet assumes another form, not by admixture, but by a self-caused change, what will it first become, bitter, or salt, or astringent, or acid? I think acid. Therefore where sweet humour is the least suitable of all, acid humour is the next least suitable to be administered. ¹ If a man can in this way conduct with success inquiries outside the human body, he will always be able to select the very best treatment. And the best is always that which is farthest removed from the unsuitable.