Custom: the feeling of facility

Of the three factors involved in causal inferences – remembered constant conjunction, custom, and belief – the one on which we have yet to focus is custom. Customary association is the centerpiece of Hume's theory of causation, just as of so much else in his philosophy:

Now as we call every thing CUSTOM, which proceeds from a past repetition, without any new reasoning or conclusion, we may establish it as a certain truth, that all the belief which follows upon any present impression, is deriv'd solely from that origin. (T102)

According to my system, all reasonings are nothing but the effects of custom; and custom has no influence, but by inlivening the imagination, and giving us a strong conception of any object. (T149) Custom has two original effects upon the mind, in bestowing a facility in the performance of any action or the conception of any object; and afterwards a tendency or inclination towards it; and from these we may account for all its other effects, however extraordinary. (T422)