Now these various dispositions and measures of health are by way of excess and defect:  wherefore if the name of health were given to the most perfect measure, then we should not speak of health as greater or less.  Thus therefore it is clear how a quality or form may increase or decrease of itself, and how it cannot.

But if we consider a quality or form in respect of its participation by the subject, thus again we find that some qualities and forms are susceptible of more or less, and some not.  Now Simplicius assigns the cause of this diversity to the fact that substance in itself cannot be susceptible of more or less, because it is per se being.  And therefore every form which is participated substantially by its subject, cannot vary in intensity and remission:  wherefore in the genus of substance nothing is said to be more or less.  And because quantity is nigh to substance, and because shape follows on quantity, therefore is it that neither in these can there be such a thing as more or less.  Whence the Philosopher says (Phys. vii, text. 15) that when a thing receives form and shape, it is not said to be altered, but to be made.  But other qualities which are further removed from quantity, and are connected with passions and actions, are susceptible of more or less, in respect of their participation by the subject.

Now it is possible to explain yet further the reason of this diversity.  For, as we have said, that from which a thing receives its species must remain indivisibly fixed and constant in something indivisible.  Wherefore in two ways it may happen that a form cannot be participated more or less.  First because the participator has its species in respect of that form.  And for this reason no substantial form is participated more or less.  Wherefore the Philosopher says (Metaph. viii, text. 10) that, “as a number cannot be more or less, so neither can that which is in the species of substance,” that is, in respect of its participation of the specific form:  “but in so far as substance may be with matter,” i.e. in respect of material dispositions, “more or less are found in substance.”

Secondly this may happen from the fact that the form is essentially indivisible:  wherefore if anything participate that form, it must needs participate it in respect of its indivisibility.  For this reason we do not speak of the species of number as varying in respect of more or less; because each species thereof is constituted by an indivisible unity.  The same is to be said of the species of continuous quantity, which are denominated from numbers, as two-cubits-long, three-cubits-long, and of relations of quantity, as double and treble, and of figures of quantity, as triangle and tetragon.

This same explanation is given by Aristotle in the Predicaments (Categor. vi), where in explaining why figures are not susceptible of more or less, he says:  “Things which are given the nature of a triangle or a circle, are accordingly triangles and circles”:  to wit, because indivisibility is essential to the motion of such, wherefore whatever participates their nature must participate it in its indivisibility.