_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 Obj. 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 Obj. 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 Obj. 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.
_______________________

SIXTH ARTICLE [II-II, Q. 53, Art. 6]

Whether the Aforesaid Vices Arise from Lust?

Objection 1: It would seem that the aforesaid vices do not arise from
lust. For inconstancy arises from envy, as stated above (A. 5, ad 2).
But envy is a distinct vice from lust.

Obj. 2: Further, it is written (James 1:8): "A double-minded man is
inconstant in all his ways." Now duplicity does not seem t