6. This connexion made by custom.
This strong combination of ideas, not allied by nature, the mind makes in
itself either voluntarily or by chance; and hence it comes in different men to be very different, according to their
different inclinations, education, interests, etc. Custom settles habits of thinking in the understanding, as well as of
determining in the will, and of motions in the body: all which seems to be but trains of motions in the animal
spirits, which, once set a going, continue in the same steps they have used to; which, by often treading, are worn
into a smooth path, and the motion in it becomes easy, and as it were natural. As far as we can comprehend
thinking, thus ideas seem to be produced in our minds; or, if they are not, this may serve to explain their following
one another in an habitual train, when once they are put into their track, as well as it does to explain such motions
of the body. A musician used to any tune will find that, let it but once begin in his head, the ideas of the several
notes of it will follow one another orderly in his understanding, without any care or attention, as regularly as his
fingers move orderly over the keys of the organ to play out the tune he has begun, though his unattentive thoughts
be elsewhere a wandering. Whether the natural cause of these ideas, as well as of that regular dancing of his
fingers be the motion of his animal spirits, I will not determine, how probable soever, by this instance, it appears
to be so: but this may help us a little to conceive of intellectual habits, and of the tying together of ideas.
