Therefore we must explain otherwise the distinction of dispositions and habits from other qualities.  For quality, properly speaking, implies a certain mode of substance.  Now mode, as Augustine says (Gen. ad lit. iv, 3), “is that which a measure determines”:  wherefore it implies a certain determination according to a certain measure.  Therefore, just as that in accordance with which the material potentiality (potentia materiae) is determined to its substantial being, is called quality, which is a difference affecting the substance, so that, in accordance with the potentiality of the subject is determined to its accidental being, is called an accidental quality, which is also a kind of difference, as is clear from the Philosopher (Metaph. v, text. 19).

Now the mode o[r] determination of the subject to accidental being may be taken in regard to the very nature of the subject, or in regard to action, and passion resulting from its natural principles, which are matter and form; or again in regard to quantity.  If we take the mode or determination of the subject in regard to quantity, we shall then have the fourth species of quality.  And because quantity, considered in itself, is devoid of movement, and does not imply the notion of good or evil, so it does not concern the fourth species of quality whether a thing be well or ill disposed, nor quickly or slowly transitory.

But the mode o[r] determination of the subject, in regard to action or passion, is considered in the second and third species of quality.  And therefore in both, we take into account whether a thing be done with ease or difficulty; whether it be transitory or lasting.  But in them, we do not consider anything pertaining to the notion of good or evil:  because movements and passions have not the aspect of an end, whereas good and evil are said in respect of an end.

On the other hand, the mode or determination of the subject, in regard to the nature of the thing, belongs to the first species of quality, which is habit and disposition:  for the Philosopher says (Phys. vii, text. 17), when speaking of habits of the soul and of the body, that they are “dispositions of the perfect to the best; and by perfect I mean that which is disposed in accordance with its nature.”  And since the form itself and the nature of a thing is the end and the cause why a thing is made (Phys. ii, text. 25), therefore in the first species we consider both evil and good, and also changeableness, whether easy or difficult; inasmuch as a certain nature is the end of generation and movement.  And so the Philosopher (Metaph. v, text. 25) defines habit, a “disposition whereby someone is disposed, well or ill”; and in Ethic. ii, 4, he says that by “habits we are directed well or ill in reference to the passions.”  For when the mode is suitable to the thing’s nature, it has the aspect of good:  and when it is unsuitable, it has the aspect of evil.  And since nature is the first object of consideration in anything, for this reason habit is reckoned as the first species of quality.