20. Comparison of Ideas in Respect of Their Comprehension. Relations of Identity and Opposition. -- Two ideas are identical or different accordingly as they have the same or a different content (the ideas of man and rational animal; of man and animal).

Of non-identical ideas some are compatible (liquid and sweet) others, incompatible (liquid and solid).

Opposition, or incompatibility, between two ideas is produced in four ways: it is contradictory, privative, contrary, relative.

(1) The opposition is contradictory when the two terms have nothing in common; for one of the terms is being, and the other is the negative of being. Two ideas, in short, are contradictory when one is neither more nor less than the negation of the other (white and not white, just and not just, etc.).

(2) Privation is the negation of a perfection in a subject which is naturally fitted to possess it (negatio alicujus formae in subjecto apto nato habere illam); thus blindness is the privation of sight, death is the privation of life. Privation is not merely synonymous with negation or absence; a mineral has no sight, but is not deprived of it.

(3) Contraries form the two extreme points of a series of elements which are joined in the same genus. Suppose, e. g., that the degrees of light are mentally arranged in a series, the two extreme terms of the series, white and black, are two contraries. There is the opposition of contrariety between things which cannot coexist in the same subject. Health and sickness, justice and injustice, courage and timidity, are contraries.

(4) Relative opposition, or relation, is that between two terms either of which needs the other to explain it. Ex. : the ideas of father and son, of double and half, of knowledge and the object known.