Habit is one of those things that everyone knows about, but few can correctly define. The common notion of it as the getting used to a thing, finding it easier to do, acquiring a taste for it, or permitting it to get a hold on one, is all very well as far as it goes, but it does not bring us very far after all. Can we hope to make things any better by invoking the aid of psychology and philosophy? Well, of course it is possible in this way to throw some light on the subject, and especially to render it more interesting by linking it up with the more attractive problems of these sciences; but when all is said and done, our ignorance of important aspects of the question will, from the very nature of the case, still largely outweigh our knowledge.

St. Thomas, following Aristotle, has defined habit as “A quality, not easily admitting of change, which modifies its subject, either in the sense of improving or impairing it, in its inner constitution or in the exercise of its activities.” Like all the definitions of Aristotle, this one, for all that it has a rather commonplace air, possesses a really vast depth of meaning.

A habit must not, however, be regarded merely as the sum total of a series of impressions, but rather as the unique and indivisible resultant of them all. It bears an analogy, not so much to a cinema film, with its graduated series of distinct though almost similar photographs, which, when flashed rapidly across the screen, produce the illusion of a continuous motion, as to a composite photograph, the distilled essence of several exposures combined.