Objection 1. It would seem that evil of nature is not an object of fear. For the Philosopher says (Rhet. ii, 5) that "fear makes us take counsel." But we do not take counsel about things which happen naturally, as stated in Ethic. iii, 3. Therefore evil of nature is not an object of fear.
Objection 2. Further, natural defects such as death and the like are always threatening man. If therefore such like evils were an object of fear, man would needs be always in fear.
Objection 3. Further, nature does not move to contraries. But evil of nature is an effect of nature. Therefore if a man shrinks from such like evils through fear thereof, this is not an effect of nature. Therefore natural fear is not of the evil of nature; and yet it seems that it should be.
On the contrary, The Philosopher says (Ethic. iii, 6) that "the most terrible of all things is death," which is an evil of nature.
I answer that, As the Philosopher says (Rhet. ii, 5), fear is caused by the "imagination of a future evil which is either corruptive or painful." Now just as a painful evil is that which is contrary to the will, so a corruptive evil is that which is contrary to nature: and this is the evil of nature. Consequently evil of nature can be the object of fear.
But it must be observed that evil of nature sometimes arises from a natural cause; and then it is called evil of nature, not merely from being a privation of the good of nature, but also from being an effect of nature; such are natural death and other like defects. But sometimes evil of nature arises from a non-natural cause; such as violent death inflicted by an assailant. In either case evil of nature is feared to a certain extent, and to a certain extent not. For since fear arises "from the imagination of future evil," as the Philosopher says (Rhet. ii, 5), whatever removes the imagination of the future evil, removes fear also. Now it may happen in two ways that an evil may not appear as about to be. First, through being remote and far off: for, on account of the distance, such a thing is considered as though it were not to be. Hence we either do not fear it, or fear it but little; for, as the Philosopher says (Rhet. ii, 5), "we do not fear things that are very far off; since all know that they shall die, but as death is not near, they heed it not." Secondly, a future evil is considered as though it were not to be, on account of its being inevitable, wherefore we look upon it as already present. Hence the Philosopher says (Rhet. ii, 5) that "those who are already on the scaffold, are not afraid," seeing that they are on the very point of a death from which there is no escape; "but in order that a man be afraid, there must be some hope of escape for him."
Consequently evil of nature is not feared if it be not apprehended as future: but if evil of nature, that is corruptive, be apprehended as near at hand, and yet with some hope of escape, then it will be feared.
Reply to Objection 1. The evil of nature sometimes is not an effect of nature, as stated above. But in so far as it is an effect of nature, although it may be impossible to avoid it entirely, yet it may be possible to delay it. And with this hope one may take counsel about avoiding it.
Reply to Objection 2. Although evil of nature ever threatens, yet it does not always threaten from near at hand: and consequently it is not always feared.
Reply to Objection 3. Death and other defects of nature are the effects of the common nature; and yet the individual nature rebels against them as far as it can. Accordingly, from the inclination of the individual nature arise pain and sorrow for such like evils, when present; fear when threatening in the future.
Source: Summa Theologica (New Advent)