INCOSTANZA. - From the Latin inconstantia, lack of steadfastness, fickleness, it is a vice opposed to prudence, insofar as it leads one to change one’s judgment without reason. The initial motive for such a change lies in the appetite, which impels one to depart from one’s first good resolution in view of something that is loved in a disordered manner; but its true cause lies in a defect of reason, which rejects what it had previously accepted, through weakness in resisting the impulse of passion, which it could and should resist.
It follows that inconstancy is a defect of reason that fails in commanding what has already been deliberated and judged. Inconstancy, therefore, is a vice contrary to prudence by defect, and more properly to that activity of prudence (imperium) by which the intellect proposes its judgment to the will as that which must be carried out (Sum. Theol., 2a-2ac, q. 47, a. 8; q. 53, a. 5).
Inconstancy is also an inward vice proceeding from lust, insofar as a man, driven by concupiscence and love of sensual things, is prevented from carrying out what he had decided to do; for example, from withdrawing from a proximate occasion of sin. It is the vice of that man who vainly promised to withdraw from the dangerous person, and to whom Terence (Eun. 67-9) repeated reproachfully: Haec verba una... falsa lacrimula... restinguet (cf. Sum. Theol., 2a-2ac, q. 153, a. 5).
Summa theologiae moralis, II, 5th ed., Parigi 1947, nos. 37 and 38, 929; W. Iankélévitch, Traité des vertus, ibid. 1949, pp. 608-20. Andrea Gennaro