Book I chapter 29

In the case of any refutations whose reasoning depends on some addition, look and see if upon its subtraction¹ the absurdity² follows none the less: and then if so, the answerer should point this out, and say that he granted the addition not because he really thought it, but for the sake of the argument, whereas the questioner has not used it for the purpose of his argument at all.

1 subtraction   

n.减法，减去

参考例句：

• We do addition and subtraction in arithmetic.在算术里,我们作加减运算。
• They made a subtraction of 50 dollars from my salary.他们从我的薪水里扣除了五十美元。

2 absurdity   

n.荒谬，愚蠢；谬论

参考例句：

• The proposal borders upon the absurdity.这提议近乎荒谬。
• The absurdity of the situation made everyone laugh.情况的荒谬可笑使每个人都笑了。