Avicenna in the Canon of Medicine said,
“Some think that strength of body depends on abundance of blood; that weakness is associated with paucity of blood. But it is not so.”
“It is rather this, that the state of the body determines whether the nutriment will be beneficial to it or not.”
“Others again, believe that whether the humours be increased or lessened in amount, the maintenance of health depends on the preservation of a certain quantitative proportion between the several humours, one to another, peculiar to the human body. But that is not exactly correct.”
“The humours must, besides that, maintain a certain constant quantity. It is not a matter of the composition of one or other humour, but of (the body) itself; but the proportions which they bear one to another must also be preserved.”