Now being differs from becoming, as the cause from the effect, the father from the son. For the same thing cannot both be and become at the same instant; and consequently it is not the cause of itself. Things are not causes of one another, but causes to each other. For the splenetic affection preceding is not the cause of fever, but of the occurrence of fever; and the fever which precedes is not the cause of spleen, but of the affection increasing.