There is a distinction between positive magnitude and ideal magnitude, water and ideal water, and so between many but yet not all substances, as with some the two states are identical, but the mind judges of flesh and ideal flesh either by some different faculty or by being itself differently disposed; for flesh cannot be without matter, but, as is a snub nose, it is something in something. Now, it is by the sensibility that we judge of hot and cold and other properties of which flesh is the standard; but it is either by some distinct faculty or as a curved is to an extended line, that we judge of ideal flesh. Straightness, on the other hand, as well as the snub nose we place among abstractions, for each is associated with continuity; but the difference, if there be a difference, between positive straightness and ideal straightness, the mind judges of by some other, perhaps a dual faculty; by some other faculty, at least, or by being itself differently disposed. To use a general expression, as are things abstracted from matter so are subjects of thought with respect to the mind.
It is difficult to determine how the mind, if it be as Anaxagoras supposes, homogeneous, impassive and without any thing in common with aught else, is to think, if thinking be some kind of impression; for it is only in so far as there is something in common between two substances, that the one seems to act and the other to be acted upon. And there is