Chapter 12

As these distinctions have been made, we must consider the mutual¹ relation of those affirmations and denials which assert or deny possibility or contingency², impossibility or necessity: for the subject is not without difficulty.

We admit that of composite expressions those are contradictory³ each to each which have the verb ‘to be’ its positive and negative form respectively. Thus the contradictory of the proposition ‘man is’ is ‘man is not’, not ‘not-man is’, and the contradictory of ‘man is white’ is ‘man is not white’, not ‘man is not-white’. For otherwise, since either the positive or the negative proposition is true of any subject, it will turn out true to say that a piece of wood is a man that is not white.

Now if this is the case, in those propositions which do not contain the verb ‘to be’ the verb which takes its place will exercise the same function. Thus the contradictory of ‘man walks’ is ‘man does not walk’, not ‘not-man walks’; for to say ‘man walks’ merely equivalent to saying ‘man is walking’.

If then this rule is universal, the contradictory of ‘it may be’ is may not be’, not ‘it cannot be’.

Now it appears that the same thing both may and may not be; for instance, everything that may be cut or may walk may also escape cutting and refrain from walking; and the reason is that those things that have potentiality in this sense are not always actual. In such cases, both the positive and the negative propositions will be true; for that which is capable of walking or of being seen has also a potentiality in the opposite direction.

But since it is impossible that contradictory propositions should both be true of the same subject, it follows that’ it may not be’ is not the contradictory of ‘it may be’. For it is a logical consequence of what we have said, either that the same predicate can be both applicable and inapplicable to one and the same subject at the same time, or that it is not by the addition of the verbs ‘be’ and ‘not be’, respectively, that positive and negative propositions are formed. If the former of these alternatives must be rejected, we must choose the latter.

The contradictory, then, of ‘it may be’ is ‘it cannot be’. The same rule applies to the proposition ‘it is contingent⁴ that it should be’; the contradictory of this is ‘it is not contingent that it should be’. The similar propositions, such as ‘it is necessary’ and ‘it is impossible’, may be dealt with in the same manner. For it comes about that just as in the former instances the verbs ‘is’ and ‘is not’ were added to the subject-matter of the sentence ‘white’ and ‘man’, so here ‘that it should be’ and ‘that it should not be’ are the subject-matter and ‘is possible’, ‘is contingent’, are added. These indicate that a certain thing is or is not possible, just as in the former instances ‘is’ and ‘is not’ indicated that certain things were or were not the case.

The contradictory, then, of ‘it may not be’ is not ‘it cannot be’, but ‘it cannot not be’, and the contradictory of ‘it may be’ is not ‘it may not be’, but cannot be’. Thus the propositions ‘it may be’ and ‘it may not be’ appear each to imply the other: for, since these two propositions are not contradictory, the same thing both may and may not be. But the propositions ‘it may be’ and ‘it cannot be’ can never be true of the same subject at the same time, for they are contradictory. Nor can the propositions ‘it may not be’ and ‘it cannot not be’ be at once true of the same subject.

The propositions which have to do with necessity are governed by the same principle. The contradictory of ‘it is necessary that it should be’, is not ‘it is necessary that it should not be,’ but ‘it is not necessary that it should be’, and the contradictory of ‘it is necessary that it should not be’ is ‘it is not necessary that it should not be’.

Again, the contradictory of ‘it is impossible that it should be’ is not ‘it is impossible that it should not be’ but ‘it is not impossible that it should be’, and the contradictory of ‘it is impossible that it should not be’ is ‘it is not impossible that it should not be’.

To generalize, we must, as has been stated, define the clauses ‘that it should be’ and ‘that it should not be’ as the subject-matter of the propositions, and in making these terms into affirmations and denials we must combine them with ‘that it should be’ and ‘that it should not be’ respectively.

We must consider the following pairs as contradictory propositions:
It may be.     It cannot be.
It is contingent.     It is not contingent.
It is impossible.     It is not impossible.
It is necessary.     It is not necessary.
It is true.     It is not true.

1 mutual   

adj.相互的，彼此的；共同的，共有的

参考例句：

• We must pull together for mutual interest.我们必须为相互的利益而通力合作。
• Mutual interests tied us together.相互的利害关系把我们联系在一起。

2 contingency   

n.意外事件，可能性

参考例句：

• We should be prepared for any contingency.我们应该对任何应急情况有所准备。
• A fire in our warehouse was a contingency that we had not expected.库房的一场大火是我们始料未及的。

3 contradictory   

adj.反驳的，反对的，抗辩的；n.正反对，矛盾对立

参考例句：

• The argument is internally contradictory.论据本身自相矛盾。
• What he said was self-contradictory.他讲话前后不符。

4 contingent   

adj.视条件而定的；n.一组，代表团，分遣队

参考例句：

• The contingent marched in the direction of the Western Hills.队伍朝西山的方向前进。
• Whether or not we arrive on time is contingent on the weather.我们是否按时到达要视天气情况而定。