Objection 1. It would seem that every circumstance relating to good or evil, specifies an action. For good and evil are specific differences of moral actions. Therefore that which causes a difference in the goodness or malice of a moral action, causes a specific difference, which is the same as to make it differ in species. Now that which makes an action better or worse, makes it differ in goodness and malice. Therefore it causes it to differ in species. Therefore every circumstance that makes an action better or worse, constitutes a species.
Objection 2. Further, an additional circumstance either has in itself the character of goodness or malice, or it has not. If not, it cannot make the action better or worse; because what is not good, cannot make a greater good; and what is not evil, cannot make a greater evil. But if it has in itself the character of good or evil, for this very reason it has a certain species of good or evil. Therefore every circumstance that makes an action better or worse, constitutes a new species of good or evil.
Objection 3. Further, according to Dionysius (Div. Nom. iv), "evil is caused by each single defect." Now every circumstance that increases malice, has a special defect. Therefore every such circumstance adds a new species of sin. And for the same reason, every circumstance that increases goodness, seems to add a new species of goodness: just as every unity added to a number makes a new species of number; since the good consists in "number, weight, and measure" (I, 5, 5).
On the contrary, More and less do not change a species. But more and less is a circumstance of additional goodness or malice. Therefore not every circumstance that makes a moral action better or worse, places it in a species of good or evil.
I answer that, As stated above (Article 10), a circumstance gives the species of good or evil to a moral action, in so far as it regards a special order of reason. Now it happens sometimes that a circumstance does not regard a special order of reason in respect of good or evil, except on the supposition of another previous circumstance, from which the moral action takes its species of good or evil. Thus to take something in a large or small quantity, does not regard the order of reason in respect of good or evil, except a certain other condition be presupposed, from which the action takes its malice or goodness; for instance, if what is taken belongs to another, which makes the action to be discordant with reason. Wherefore to take what belongs to another in a large or small quantity, does not change the species of the sin. Nevertheless it can aggravate or diminish the sin. The same applies to other evil or good actions. Consequently not every circumstance that makes a moral action better or worse, changes its species.
Reply to Objection 1. In things which can be more or less intense, the difference of more or less does not change the species: thus by differing in whiteness through being more or less white a thing is not changed in regard to its species of color. In like manner that which makes an action to be more or less good or evil, does not make the action differ in species.
Reply to Objection 2. A circumstance that aggravates a sin, or adds to the goodness of an action, sometimes has no goodness or malice in itself, but in regard to some other condition of the action, as stated above. Consequently it does not add a new species, but adds to the goodness or malice derived from this other condition of the action.
Reply to Objection 3. A circumstance does not always involve a distinct defect of its own; sometimes it causes a defect in reference to something else. In like manner a circumstance does not always add further perfection, except in reference to something else. And, for as much as it does, although it may add to the goodness or malice, it does not always change the species of good or evil.
Source: Summa Theologica (New Advent)