Objection 1. It would seem that defect is not a cause of fear. Because those who are in power are very much feared. But defect is contrary to power. Therefore defect is not a cause of fear.
Objection 2. Further, the defect of those who are already being executed is extreme. But such like do not fear as stated in Rhet. ii, 5. Therefore defect is not a cause of fear.
Objection 3. Further, contests arise from strength not from defect. But "those who contend fear those who contend with them" (Rhet. ii, 5). Therefore defect is not a cause of fear.
On the contrary, Contraries ensue from contrary causes. But "wealth, strength, a multitude of friends, and power drive fear away" (Rhet. ii, 5). Therefore fear is caused by lack of these.
I answer that, As stated above (Article 1), fear may be set down to a twofold cause: one is by way of a material disposition, on the part of him that fears; the other is by way of efficient cause, on the part of the person feared. As to the first then, some defect is, of itself, the cause of fear: for it is owing to some lack of power that one is unable easily to repulse a threatening evil. And yet, in order to cause fear, this defect must be according to a measure. For the defect which causes fear of a future evil, is less than the defect caused by evil present, which is the object of sorrow. And still greater would be the defect, if perception of the evil, or love of the good whose contrary is feared, were entirely absent.
But as to the second, power and strength are, of themselves, the cause of fear: because it is owing to the fact that the cause apprehended as harmful is powerful, that its effect cannot be repulsed. It may happen, however, in this respect, that some defect causes fear accidentally, in so far as owing to some defect someone wishes to hurt another; for instance, by reason of injustice, either because that other has already done him a harm, or because he fears to be harmed by him.
Reply to Objection 1. This argument is true of the cause of fear, on the part of the efficient cause.
Reply to Objection 2. Those who are already being executed, are actually suffering from a present evil; wherefore their defect exceeds the measure of fear.
Reply to Objection 3. Those who contend with one another are afraid, not on account of the power which enables them to contend: but on account of the lack of power, owing to which they are not confident of victory.
Source: Summa Theologica (New Advent)