By degrees the repetition produces a facility of the human mind, and an infallible source of pleasure, where the facility goes not beyond a certain degree.  And here it is remarkable that the pleasure, which arises from a moderate facility, has not the same tendency with that which arises from novelty, to augment the painful, as well as the agreeable affections.  The pleasure of facility does not so much consist in any ferment of the spirits, as in their orderly motion; which will sometimes be so powerful as even to convert pain into pleasure, and give us a relish in time what at first was most harsh and disagreeable.

But again, as facility converts pain into pleasure, so it often converts pleasure into pain, when it is too great, and renders the actions of the mind so faint and languid, that they are no longer able to interest and support it.  And indeed, scarce any other objects become disagreeable through custom; but such as are naturally attended with some emotion or affection, which is destroyed by the too frequent repetition.  One can consider the clouds, and heavens, and trees, and stones, however frequently repeated, without ever feeling any aversion.  But when the fair sex, or music, or good cheer, or any thing, that naturally ought to be agreeable, becomes indifferent, it easily produces the opposite affection.

But custom not only gives a facility to perform any action, but likewise an inclination and tendency towards it, where it is not entirely disagreeable, and can never be the object of inclination.  And this is the reason why custom encreases all active habits, but diminishes passive, according to the observation of a late eminent philosopher.  The facility takes off from the force of the passive habits by rendering the motion of the spirits faint and languid.  But as in the active, the spirits are sufficiently supported of themselves, the tendency of the mind gives them new force, and bends them more strongly to the action.

SECT.  VI OF THE INFLUENCE OF THE IMAGINATION ON THE PASSIONS

It is remarkable, that the imagination and affections have close union together, and that nothing, which affects the former, can be entirely indifferent to the latter.  Wherever our ideas of good or evil acquire a new vivacity, the passions become more violent; and keep pace with the imagination in all its variations.  Whether this proceeds from the principle above-mentioned, that any attendant emotion is easily converted into the predominant, I shall not determine.  It is sufficient for my present purpose, that we have many instances to confirm this influence of the imagination upon the passions.