[...] men are not astonish'd at the operations of their own reason, at the same time, that they admire the instinct of animals, and find a difficulty in explaining it, merely because it cannot be reduc'd to the very same principles. [...] reason is nothing but a wonderful and unintelligible instinct in our souls [...] Nay, habit is nothing but one of the principles of nature, and derives all its force from that origin.

§ 3.16 - A Treatise of Human Nature (1739-40) - Of The Understanding