Objection 1: It would seem that sorrow does not weaken all activity. Because carefulness is caused by sorrow, as is clear from the passage of the Apostle quoted above (A. 2, Obj. 1). But carefulness conduces to good work: wherefore the Apostle says (2 Tim. 2:15): "Carefully study to present thyself . . . a workman that needeth not to be ashamed." Therefore sorrow is not a hindrance to work, but helps one to work well.
Obj. 2: Further, sorrow causes desire in many cases, as stated in Ethic. vii, 14. But desire causes intensity of action. Therefore sorrow does too.
Obj. 3: Further, as some actions are proper to the joyful, so are others proper to the sorrowful; for instance, to mourn. Now a thing is improved by that which is suitable to it. Therefore certain actions are not hindered but improved by reason of sorrow.
On the contrary, The Philosopher says (Ethic. x, 4) that "pleasure perfects action," whereas on the other hand, "sorrow hinders it" (Ethic. x, 5).
I answer that, As stated above (A. 2), sorrow at times does not depress or consume the soul, so as to shut out all movement, internal or external; but certain movements are sometimes caused by sorrow itself. Accordingly action stands in a twofold relation to sorrow. First, as being the object of sorrow: and thus sorrow hinders any action: for we never do that which we do with sorrow, so well as that which we do with pleasure, or without sorrow. The reason for this is that the will is the cause of human actions: and consequently when we do something that gives pain, the action must of necessity be weakened in consequence. Secondly, action stands in relation to sorrow, as to its principle and cause: and such action must needs be improved by sorrow: thus the more one sorrows on account of a certain thing, the more one strives to shake off sorrow, provided there is a hope of shaking it off: otherwise no movement or action would result from that sorrow.
From what has been said the replies to the objections are evident.