Summa summae S. Thomae, Tomus II

The difficulty concerns the conflict of manifold moralities opposed one to another, whereof one namely is good, another evil; or whether the same act can be at once good and evil from diverse heads, good from object, evil from end or some other circumstance, and conversely.

I say 1°: An act otherwise good in itself, whose evil end is the total and adequate reason why it is chosen, is from no part good, but wholly evil. It is the common teaching ¹

It is proved 1° from Scripture. I Cor., v, 6, the Apostle says: A little leaven corrupts the whole lump. By leaven moreover is understood the depraved intention, which while it is the total cause of an act otherwise good, infects and corrupts the whole of it.

It is proved 2° by reason. When one gives alms precisely for vainglory or luxury, so that vainglory or luxury be the total and adequate cause of bestowing the alms, the honesty of the alms is nowise desired, but desired only as the means of attaining glory or luxury; so that, even if it had no honesty of itself, it would nevertheless be assumed for attaining the evil end. But the honesty of the object reflects no goodness into the act unless directly desired in itself, as was said before. Therefore.

I say 2°: An act otherwise good in itself, whose evil end is not the total and adequate reason why it is chosen, can be good from object and evil from end. A fortiori an act good from object and end, upon which supervenes some venially evil circumstance, can be partly good and partly evil ²

It is proved. An act good whose extrinsic evil end is not the total and adequate cause why it is assumed, e.g., alms done partly on account of one's own honesty of mercy, partly for vainglory, draws and retains goodness from the object, notwithstanding the concurrence of the further extrinsic end, evil namely, of vainglory. Therefore it is good from object and evil from end. — Proof of the antecedent: It cannot be but that the goodness of the object willed in itself and for itself reflect goodness into the act. For to love the good in itself and for itself is necessarily and essentially good. But honest alms, in the case, is loved in itself and for itself, and not precisely and solely for vainglory, which we suppose not to be its total and adequate cause. Therefore.

What I said of the act good from object and evil from end must be said a fortiori of the act good from object and end, into which mingles some venially evil circumstance, e.g., if into alms done from the sole motive of mercy there mingle tepidity or inconsiderateness.