… is from pages 56-57 of F.A. Hayek’s 1962 article “Rules, Perception and Intelligibility,” which is reprinted as chapter 3 in Hayek’s 1967 collection, Studies in Philosophy, Politics, and Economics  (footnote excluded):
By eliminating certain kinds of action altogether and providing certain routine ways of achieving the object, [the unconscious rules which govern our action] merely restrict the alternatives on which a conscious choice is required. The moral rules, for example, which have become part of man’s nature will mean that certain conceivable choices will not appear at all among the possibilities between which he chooses. Thus even decisions which have been carefully considered will in part be determined by rules of which the acting person is not aware. Like scientific laws, the rules which guide an individual’s action are better seen as determining what he will not do rather than what he will do.