Objection 1. It would seem that fear hinders action. For action is hindered chiefly by a disturbance in the reason, which directs action. But fear disturbs reason, as stated above (Article 2). Therefore fear hinders action.
Objection 2. Further, those who fear while doing anything, are more apt to fail: thus a man who walks on a plank placed aloft, easily falls through fear; whereas, if he were to walk on the same plank down below, he would not fall, through not being afraid. Therefore fear hinders action.
Objection 3. Further, laziness or sloth is a kind of fear. But laziness hinders action. Therefore fear does too.
On the contrary, The Apostle says (Philippians 2:12): "With fear and trembling work out your salvation": and he would not say this if fear were a hindrance to a good work. Therefore fear does not hinder a good action.
I answer that, Man's exterior actions are caused by the soul as first mover, but by the bodily members as instruments. Now action may be hindered both by defect of the instrument, and by defect of the principal mover. On the part of the bodily instruments, fear, considered in itself, is always apt to hinder exterior action, on account of the outward members being deprived, through fear, of their heat. But on the part of the soul, if the fear be moderate, without much disturbance of the reason, it conduces to working well, in so far as it causes a certain solicitude, and makes a man take counsel and work with greater attention. If, however, fear increases so much as to disturb the reason, it hinders action even on the part of the soul. But of such a fear the Apostle does not speak.
This suffices for the Reply to the First Objection.
Reply to Objection 2. He that falls from a plank placed aloft, suffers a disturbance of his imagination, through fear of the fall that is pictured to his imagination.
Reply to Objection 3. Everyone in fear shuns that which he fears: and therefore, since laziness is a fear of work itself as being toilsome, it hinders work by withdrawing the will from it. But fear of other things conduces to action, in so far as it inclines the will to do that whereby a man escapes from what he fears.
Source: Summa Theologica (New Advent)