Objection 1. It would seem that inconstancy is not a vice contained under imprudence. For inconstancy consists seemingly in a lack of perseverance in matters of difficulty. But perseverance in difficult matters belongs to fortitude. Therefore inconstancy is opposed to fortitude rather than to prudence.
Objection 2. Further, it is written (James 3:16): "Where jealousy [Douay: 'envy'] and contention are, there are inconstancy and every evil work."
But jealousy pertains to envy. Therefore inconstancy pertains not to imprudence but to envy.
Objection 3. Further, a man would seem to be inconstant who fails to persevere in what he has proposed to do. Now this is a mark of "incontinency" in pleasurable matters, and of "effeminacy" or "squeamishness" in unpleasant matters, according to Ethic. vii, 1. Therefore inconstancy does not pertain to imprudence.
On the contrary, It belongs to prudence to prefer the greater good to the lesser. Therefore to forsake the greater good belongs to imprudence. Now this is inconstancy. Therefore inconstancy belongs to imprudence.
I answer that, Inconstancy denotes withdrawal from a definite good purpose. Now the origin of this withdrawal is in the appetite, for a man does not withdraw from a previous good purpose, except on account of something being inordinately pleasing to him: nor is this withdrawal completed except through a defect of reason, which is deceived in rejecting what before it had rightly accepted. And since it can resist the impulse of the passions, if it fail to do this, it is due to its own weakness in not standing to the good purpose it has conceived; hence inconstancy, as to its completion, is due to a defect in the reason. Now just as all rectitude of the practical reason belongs in some degree to prudence, so all lack of that rectitude belongs to imprudence. Consequently inconstancy, as to its completion, belongs to imprudence. And just as precipitation is due to a defect in the act of counsel, and thoughtlessness to a defect in the act of judgment, so inconstancy arises from a defect in the act of command. For a man is stated to be inconstant because his reason fails in commanding what has been counselled and judged.
Reply to Objection 1. The good of prudence is shared by all the moral virtues, and accordingly perseverance in good belongs to all moral virtues, chiefly, however, to fortitude, which suffers a greater impulse to the contrary.
Reply to Objection 2. Envy and anger, which are the source of contention, cause inconstancy on the part of the appetite, to which power the origin of inconstancy is due, as stated above.
Reply to Objection 3. Continency and perseverance seem to be not in the appetitive power, but in the reason. For the continent man suffers evil concupiscences, and the persevering man suffers grievous sorrows (which points to a defect in the appetitive power); but reason stands firm, in the continent man, against concupiscence, and in the persevering man, against sorrow. Hence continency and perseverance seem to be species of constancy which pertains to reason; and to this power inconstancy pertains also.
Source: Summa Theologica (New Advent)