The WILL exerts itself, when either the good or the absence of the evil may be attained by any action of the mind or body.
Beside good and evil, or in other words, pain and pleasure, the direct passions frequently arise from a natural impulse or instinct, which is perfectly unaccountable.

Of this kind is the desire of punishment to our enemies, and of happiness to our friends; hunger, lust, and a few other bodily appetites.

These passions, properly speaking, produce good and evil, and proceed not from them, like the other affections.
None of the direct affections seem to merit our particular attention, except hope and fear, which we shall here endeavour to account for.
It is evident that the very same event, which by its certainty would produce grief or joy, gives always rise to fear or hope, when only probable and uncertain.

In order, therefore, to understand the reason why this circumstance makes such a considerable difference, we must reflect on what I have already advanced in the preceding book concerning the nature of probability.
Probability arises from an opposition of contrary chances or causes, by which the mind is not allowed to fix on either side, but is incessantly tost from one to another, and at one moment is determined to consider an object as existent, and at another moment as the contrary.

The imagination or understanding, call it which you please, fluctuates betwixt the opposite views; and though perhaps it may be oftener turned to the one side than the other, it is impossible for it, by reason of the opposition of causes or chances, to rest on either.