When we see a person free from these motives, we suppose a possibility either of his acting or forbearing; and though in general we may conclude him to be determined by motives and causes, yet this removes not the uncertainty of our judgment concerning these causes, nor the influence of that uncertainty on the passions.

Since therefore we ascribe a power of performing an action to every one, who has no very powerful motive to forbear it, and refuse it to such as have; it may justly be concluded, that power has always a reference to its exercise, either actual or probable, and that we consider a person as endowed with any ability when we find from past experience, that it is probable, or at least possible he may exert it.

And indeed, as our passions always regard the real existence of objects, and we always judge of this reality from past instances; nothing can be more likely of itself, without any farther reasoning, than that power consists in the possibility or probability of any action, as discovered by experience and the practice of the world.
Now it is evident, that wherever a person is in such a situadon with regard to me, that there is no very powerful motive to deter him from injuring me, and consequently it is uncertain whether he will injure me or not, I must be uneasy in such a situation, and cannot consider the possibility or probability of that injury without a sensible concern.
The passions are not only affected by such events as are certain and infallible, but also in an inferior degree by such as are possible and contingent.

And though perhaps I never really feel any harm, and discover by the event, that, philosophically speaking, the person never had any power of harming me; since he did not exert any; this prevents not my uneasiness from the preceding uncertainty.

The agreeable passions may here operate as well as the uneasy, and convey a pleasure when I perceive a good to become either possible or probable by the possibility or probability of another's bestowing it on me, upon the removal of any strong motives, which might formerly have hindered him.
But we may farther observe, that this satisfaction encreases, when any good approaches in such a manner that it it in one's own power to take or leave it, and there neither is any physical impediment, nor any very strong motive to hinder our enjoyment.