Cursus Philosophicus Thomisticus, Secunda Pars Artis Logicae

Aristotle defines quality in ch. 8¹ as “that by which we are said to be qualified.” Some simply reject this definition: first, because it seems tautological, as if whiteness were said to be that by which we are said to be white; but that is just what needs to be explained. Second, because it seems circular: just as quality is defined by our being qualified, so, conversely, a qualified thing is defined as that which has a quality, which is to commit a circle. Finally, because the definition is no better known than what is defined; for quality in the abstract and a qualified thing in the concrete are equally unknown.

Nevertheless, this definition should not be rejected, since it was handed down by Aristotle and received by his entire school. In the first place, since the highest genera lack a genus and a difference, this is not a definition but a description, and it is given through effects that are better known.

We can consider the effects of quality in two ways. In one way, from the standpoint of state, insofar as it is taken in the abstract or in the concrete, because the state of concretion is better known and more perceptible to us. St. Thomas explains the definition in this way in Opusculum 48, treatise on Quality, ch. 1². This way of defining, however, would not fit other definitions so well, in which the highest genera are not defined.

In a second way, the effect of quality can be considered by reason of the form itself. Among all the accidents, it is proper to quality to make a subject formed and qualified, as St. Thomas notes from the Philosopher in Metaphysics V, lect. 16³, since of all the accidents quality most properly ennobles and qualifies its subject. Quantity quantifies the subject and tends instead to materialize it by extending and ordering its material parts. The other categories either order a subject to something else, as relation does, or depend on something extrinsic that orders them, as do the last six categories, as we shall say in the next question. Quality alone is given for ennobling or qualifying a subject, or for the contrary of ennoblement and qualification. Something is said to qualify a thing through what is actual and determines it in the manner of a form. An essential difference is therefore said to be predicated in the mode of a qualification, because it contracts and determines the genus, which is potential, and by determining it forms and qualifies it. What an essential difference does essentially, quality does accidentally: namely, it forms and qualifies what is potential and unformed, and does so primarily and of itself. For although other accidents also perfect and actualize a subject, they do not have this as their primary and proper function; rather, they either quantify matter, order the subject to something extrinsic, or depend on something extrinsic for their denomination. Therefore it is rightly said that quality is that by which we are said to be qualified—that is, formed, qualified, and ennobled.

To the first objection, the answer is that this definition is not tautological and does not explain the same by the same, either because it explains the state of concretion as better known, or because it explains the special effect of this accident, which is to qualify and ennoble; by it, then, we are said to be qualified.

To the second objection, the answer is that these definitions do not form a circle, because they are not definitions but descriptions. Likewise, the abstract and the concrete stand in different genera of cause: one explains the form, the other connotes the subject. Finally, quality in the abstract is not explained only through concretion, but through the effect of ennobling and making a thing qualified. When a qualified thing is defined, however, it is defined only with regard to the state of the concrete, by its having a form. Hence no circle is committed.

To the last objection, we say that the definition is not given through something equally unknown, because it is given through the effects of qualifying and through the state of concretion in a subject, which are better known to us than the qualifying form itself or its state of abstraction. It follows less still that the thing defined is placed in its own definition, because quality in the abstract is defined by its effects, not by quality itself.

FROM THESE POINTS YOU WILL GATHER that St. Thomas expressed the account of quality better than the competing views, saying in Summa Theologiae I, q. 28, a. 2 that “quality is called a disposition of substance,” and again in I–II, q. 49, a. 2, that “quality implies a certain mode of substance, and a mode is that which measure prescribes.” Hence it implies a certain determination according to some measure. Therefore, the proper account of quality is said to consist in mode or determination insofar as “mode” is taken to mean formation and proper disposition. “Disposition” here is not taken for the first species of quality, but generally, as is said in Metaphysics V, lect. 20 according to St. Thomas⁴: “disposition is the order of that which has parts.” Hence we gather that it belongs to quantity and quality to order parts, but not the same parts or in the same way. Quantity orders material parts by way of extension, so that one is placed outside another, first in relation to itself and then in relation to place. Quality, however, orders nonmaterial parts, not by way of extension but by way of power or perfection, according to the due or undue commensuration, whether in relation to the nature itself or the termination of its quantity, in relation to an action or the term of an action, as we shall explain for each species in turn, or also in relation to the intensification of a quality or the extension of a habit. In a word, whatever involves due commensuration and proportion, whether in bodily or spiritual things, and whether with respect to parts of power or formal parts, is called a disposition by quality. Quantity, by contrast, merely places material parts outside one another, which is to extend them, not to dispose them properly and duly; for its formation with respect to termination occurs through figure, which is a quality. This seems the more probable explanation of a matter as obscure and varied as quality.