STUDY OF THE STRAIGHT LINE.
14. But what about the straight line? Is it not a magnitude? Possibly; but if it be a magnitude, it is a qualified one.[391] It is even possible that straightness constitutes a difference of the (very nature of the) line, as line, for straightness refers solely to a line; and besides, we often deduce the differences of "Essence" from its qualities. That a straight line is a quantity added to a difference does not cause its being composed of the line, and of the property of straightness; for, were it thus composed, straightness would be its chief difference.
STUDY OF THE TRIANGLE.
Now let us consider the triangle, which is formed of three lines. Why should it not belong to quantity? Would it be so, because it is not constituted by three lines merely, but by three lines arranged in some particular manner? But a quadrilateral would also be constituted by four lines arranged in some particular manner. (But being arranged in some particular manner does not hinder a figure from being a quantity). The straight line, indeed, is arranged in some particular manner, and is none the less a quantity. Now if the straight line be not simply a quantity, why could this not also be said of a limited line? For the limit of the line is a point, and the point does not belong to any genus other than the line. Consequently, a limited surface is also a quantity, because it is limited by lines, which even more belong to quantity. If then the limited surface be contained in the genus of quantity, whether the surface be a triangle, a quadrilateral, a hexagon, or any other polygon, all figures whatever will belong to the genus of quantity. But if we assigned the triangle or quadrilateral to the genus of quality merely because we are speaking of some one definite triangle or quadrilateral, nothing would hinder one and the same thing from being subsumed under several categories. A triangle would then be a quantity so far as it was both a general and particular magnitude, and would be a quality by virtue of its possessing a particular form. The same might be predicated of the Triangle in itself because of its possessing a particular form; and so also with the sphere. By following this line of argument, geometry would be turned into a study of qualities, instead of that of quantities, which of course it is. The existing differences between magnitudes do not deprive them of their property of being magnitudes, just as the difference between essences does not affect their essentiality. Besides, every surface is limited, because an infinite surface is impossible. Further, when I consider a difference that pertains to essence, I call it an essential difference. So much the more, on considering figures, I am considering differences of magnitude. For if the differences were not of magnitude, of what would they be differences? If then they be differences of magnitude, the different magnitudes which are derived from differences of magnitude should be classified according to the species constituted by them (when considered in the light of being magnitudes).
GEOMETRY STUDIES QUANTITIES, NOT QUALITIES.
15. But how can you qualify the properties of quantity so as to call them equal or unequal?[392] Is it not usual to say of two triangles that they are similar? Could we not also predicate similarity of two magnitudes? Doubtless, for what is called similarity,[393] does not conflict with similarity or dissimilarity in the genus of quantity.[394] Here, indeed, the word "similarity" is applied to magnitudes in a sense other than to quality. Besides, if (Aristotle) said that the property characteristic of quantities is to enable them to be called equal or unequal, this does not conflict with predicating similarity of some of them. But as it has been said that the special characteristic of qualities is to admit of being called similar or dissimilar, we must, as has already been explained, understand similarity in a sense other than when it is applied to magnitudes. If similar magnitudes be identical, we must then consider the other properties of quantity and quality which might be present in them (so as clearly to contrast their differences). It may also be said that the term "similarity" applies to the genus of quantity so far as this contains differences (which distinguish from each other similar magnitudes).
DIFFERENCES WHICH COMPLETE THE BEING MUST BE PREFIXED TO THAT TO WHICH THEY REFER.
In general, the differences which complete a being should be classified along with that of which they are the differences, especially when a difference belongs to a single subject. If a difference complete the being of a subject, and do not complete the being of another, this difference should be classified along with the subject whose being it completes, leaving that whose being it does not complete for separate consideration. By this we do not mean completing the Being in general, but completing some particular being, so that the subject spoken of as a particular one admits no further essential addition. We therefore have the right to say that triangles, or that quadrilaterals, as well as surfaces and solids, are equal, and to predicate equality or inequality of quantitative entities. But we yet have to study whether quality only can be said to be similar or dissimilar.[395]