It may be urged that the triangle is essentially a particular shape. Then what prevents our ranking the sphere also as a quality?
To proceed on these lines would lead us to the conclusion that geometry is concerned not with magnitudes but with Quality. But this conclusion is untenable; geometry is the study of magnitudes. The differences of magnitudes do not eliminate the existence of magnitudes as such, any more than the differences of substances annihilate the substances themselves.
Moreover, every surface is limited; it is impossible for any surface to be infinite in extent.
Again, when I find Quality bound up with Substance, I regard it as substantial quality: I am not less, but far more, disposed to see in figures or shapes [qualitative] varieties of Quantity. Besides, if we are not to regard them as varieties of magnitude, to what genus are we to assign them?
Suppose, then, that we allow differences of magnitude; we commit ourselves to a specific classification of the magnitudes so differentiated.
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