Edward Bradford Titchener, answered from the texts and cited to the page.
There are two ways to study a living thing, and they must not be confused. The morphologist describes its structure — the organs, the tissues, the elements of which it is built; the physiologist studies their function — what they do, how they serve the life of the organism. Both are sciences, but the structural inquiry is the prior one, for you cannot study the function of what you have not first described.
So it is with mind. The structural psychology I practise asks what consciousness is — its elements, their attributes, the laws of their combination — and holds this question apart from, and before, the functional question of what consciousness does for the organism that has it.1 The Americans who would leap straight to function, to the uses of mind in adaptation, are doing biology or running ahead to a physiology of mind before its anatomy is written.
I do not deny that the functional question is real; I deny that it may be asked first, or that it is the same question. Describe the structure of experience with exactness, and the study of its function has a subject; neglect the structure, and function has nothing precise to be the function of.2
'Structural' psychology, as its name implies, recognized the existence of a correlative psychology of function; it isolated itself only to the degree that it demanded equal rights for content and activity, and by so doing protested against the autocracy of function. In the 'biological' atmosphere of its time the phrase did good controversial service.Systematic Psychology Prolegomena, pp. 178–179
In another dichotomy, which is perhaps even more familiar, this relation is reversed: the universe of experience is divided into the Conscious and the Not-conscious, and psychical are identified with conscious phenomena, mind with the totality of consciousness.Systematic Psychology Prolegomena, pp. 158–159
This is the debate structuralism decisively lost. The functionalism of James, Dewey, Angell, and Carr — studying mind as an evolutionary adaptation, for its role in the organism's dealings with its environment — won the American field and opened psychology to development, individual differences, animal behavior, and application, all of which Titchener's structural program excluded. Behaviorism then carried the functional and behavioral emphasis further. The verdict of the discipline is that Titchener's insistence on structure before function narrowed psychology unduly and that the functional questions were not only legitimate but the more fruitful. Titchener's point — that description should precede explanation of use — retains a limited methodological force, but the priority he claimed for it did not hold.