"The subject was a man who is today quite indifferent to the genitals and other attractions of women, but who can be plunged into irresistible sexual excitement only by a foot of a particular form wearing a shoe. He can recall an event from his sixth year which was decisive for the fixation of his libido. He was sitting on a stool beside the governess who was to give him lessons in English. The governess, who was an elderly, dried-up, plain-looking spinster, with pale blue eyes and a snub nose, had something wrong with her foot that day, and on that account kept it, wearing a velvet slipper, stretched out on a cushion. Her leg itself was most decently concealed. A thin, scraggy foot, like the one he had then seen belonging to his governess, thereupon became (after a timid attempt at normal sexual activity at puberty) his only sexual object; and the man was irresistibly attracted if a foot of this kind was associated with other features besides which recalled the type of the English governess. This fixation of his libido, however, made him, not into a neurotic, but into a pervert - what we call a foot fetishist. You see, then, that although an excessive, and moreover premature, fixation of the libido is indispensible for the causation of neuroses, the area of its effects extends far beyond the field of the neuroses. This determinant, too, is as little decisive in itself as is the frustration which we have already talked about."
From Freud's paper on Fetishism, 1927