Objection 1. It would seem that excess and deficiency do not diversify the species of sins. For excess and deficiency differ in respect of more and less. Now "more" and "less" do not diversify a species. Therefore excess and deficiency do not diversify the species of sins.
Objection 2. Further, just as sin, in matters of action, is due to straying from the rectitude of reason, so falsehood, in speculative matters, is due to straying from the truth of the reality. Now the species of falsehood is not diversified by saying more or less than the reality. Therefore neither is the species of sin diversified by straying more or less from the rectitude of reason.
Objection 3. Further, "one species cannot be made out of two," as Porphyry declares [Isagog.; cf. Arist. Metaph. i]. Now excess and deficiency are united in one sin; for some are at once illiberal and wasteful--illiberality being a sin of deficiency, and prodigality, by excess. Therefore excess and deficiency do not diversify the species of sins.
On the contrary, Contraries differ specifically, for "contrariety is a difference of form," as stated in Metaph. x, text. 13,14. Now vices that differ according to excess and deficiency are contrary to one another, as illiberality to wastefulness. Therefore they differ specifically.
I answer that, While there are two things in sin, viz. the act itself and its inordinateness, in so far as sin is a departure from the order of reason and the Divine law, the species of sin is gathered, not from its inordinateness, which is outside the sinner's intention, as stated above (Article 1), but on the contrary, from the act itself as terminating in the object to which the sinner's intention is directed. Consequently wherever we find a different motive inclining the intention to sin, there will be a different species of sin. Now it is evident that the motive for sinning, in sins by excess, is not the same as the motive for sinning, in sins of deficiency; in fact, they are contrary to one another, just as the motive in the sin of intemperance is love for bodily pleasures, while the motive in the sin of insensibility is hatred of the same. Therefore these sins not only differ specifically, but are contrary to one another.
Reply to Objection 1. Although "more" and "less" do not cause diversity of species, yet they are sometimes consequent to specific difference, in so far as they are the result of diversity of form; thus we may say that fire is lighter than air. Hence the Philosopher says (Ethic. viii, 1) that "those who held that there are no different species of friendship, by reason of its admitting of degree, were led by insufficient proof." In this way to exceed reason or to fall short thereof belongs to sins specifically different, in so far as they result from different motives.
Reply to Objection 2. It is not the sinner's intention to depart from reason; and so sins of excess and deficiency do not become of one kind through departing from the one rectitude of reason. On the other hand, sometimes he who utters a falsehood, intends to hide the truth, wherefore in this respect, it matters not whether he tells more or less. If, however, departure from the truth be not outside the intention, it is evident that then one is moved by different causes to tell more or less; and in this respect there are different kinds of falsehood, as is evident of the "boaster," who exceeds in telling untruths for the sake of fame, and the "cheat," who tells less than the truth, in order to escape from paying his debts. This also explains how some false opinions are contrary to one another.
Reply to Objection 3. One may be prodigal and illiberal with regard to different objects: for instance one may be illiberal [Cf. II-II, 119, 1, ad 1 in taking what one ought not: and nothing hinders contraries from being in the same subject, in different respects.
Source: Summa Theologica (New Advent)