Billings on Composition

“Musical composition is a sort of something, which is much better felt  than described (at least by me) for if I was to attempt it, I should not  know where to begin or where to leave off…although I am not  confined to rules prescribed by others, yet I come near as I possibly  can to a set of rules which I have carved out for myself; but when fancy  gets upon the wing, she seems to despise all form, and scorns to be  confined or limited by any formal prescriptions whatsoever; for the  first part [tenor] is nothing more than a flight of fancy, the other  parts are forced to comply and conform to that, by partaking of the same  air, or, at least, as much of it as they can get: But by reason of this  restraint, the last parts are seldom so good as the first; for the  second part [bass] is subservient to the first, the third part [treble]  must conform to the first and second, and the fourth part [counter or  alto] must conform to the other three; therefor the grand difficulty in  composition, is to preserve the air through each part separately, and  yet cause them to harmonize with each other at the same time.”

William Billings, Continental Harmony, p. 32