This may be accounted for from the same principles, that explained the influence of general rules on the understanding.

Custom readily carries us beyond the just bounds in our passions, as well as in our reasonings.
It may not be amiss to observe on this occasion, that the influence of general rules and maxims on the passions very much contributes to facilitate the effects of all the principles, which we shall explain in the progress of this treatise.

For it is evident, that if a person full-grown, and of the same nature with ourselves, were on a sudden-transported into our world, he would be very much embarrased with every object, and would not readily find what degree of love or hatred, pride or humility, or any other passion he ought to attribute to it.

The passions are often varyed by very inconsiderable principles; and these do not always play with a perfect regularity, especially on the first trial.

But as custom and practice have brought to light all these principles, and have settled the just value of every thing; this must certainly contribute to the easy production of the passions, and guide us, by means of general established maxims, in the proportions we ought to observe in preferring one object to another.