Which of the following propositions are so related that if one is true, the other must be false, although both can be false?

Statements 1 and 3 are related so that if one is true the other must be false, yet both may be false.

Statement 1, 'All novels are books', is a universal affirmative.

Statement 3, 'No novels are books', is the universal negative on the same terms.

A universal affirmative and a universal negative are contraries, which cannot both be true but can both be false.

Statements 2 and 4 are particulars, so they do not form the contrary relation.

Hence the contrary pair is the first and the third.