Book V Chapter 5

We call ‘necessary’ (1) (a) that without which, as a condition, a thing cannot live; e.g. breathing and food are necessary for an animal; for it is incapable¹ of existing without these; (b) the conditions without which good cannot be or come to be, or without which we cannot get rid or be freed of evil; e.g. drinking the medicine is necessary in order that we may be cured of disease, and a man’s sailing to Aegina is necessary in order that he may get his money.-(2) The compulsory² and compulsion, i.e. that which impedes³ and tends to hinder, contrary to impulse and purpose. For the compulsory is called necessary (whence the necessary is painful, as Evenus says: ‘For every necessary thing is ever irksome’), and compulsion is a form of necessity, as Sophocles says: ‘But force necessitates⁴ me to this act’. And necessity is held to be something that cannot be persuaded-and rightly, for it is contrary to the movement which accords with purpose and with reasoning.-(3) We say that that which cannot be otherwise is necessarily as it is. And from this sense of ‘necessary’ all the others are somehow derived⁵; for a thing is said to do or suffer what is necessary in the sense of compulsory, only when it cannot act according to its impulse because of the compelling forces-which implies that necessity is that because of which a thing cannot be otherwise; and similarly as regards the conditions of life and of good; for when in the one case good, in the other life and being, are not possible without certain conditions, these are necessary, and this kind of cause is a sort of necessity. Again, demonstration⁶ is a necessary thing because the conclusion cannot be otherwise, if there has been demonstration in the unqualified sense; and the causes of this necessity are the first premisses, i.e. the fact that the propositions from which the syllogism⁷ proceeds cannot be otherwise.

Now some things owe their necessity to something other than themselves; others do not, but are themselves the source of necessity in other things. Therefore the necessary in the primary and strict sense is the simple; for this does not admit of more states than one, so that it cannot even be in one state and also in another; for if it did it would already be in more than one. If, then, there are any things that are eternal and unmovable, nothing compulsory or against their nature attaches to them.

1 incapable   

adj.无能力的，不能做某事的

参考例句：

• He would be incapable of committing such a cruel deed.他不会做出这么残忍的事。
• Computers are incapable of creative thought.计算机不会创造性地思维。

2 compulsory   

n.强制的，必修的；规定的，义务的

参考例句：

• Is English a compulsory subject?英语是必修课吗？
• Compulsory schooling ends at sixteen.义务教育至16岁为止。

3 impedes   

阻碍，妨碍，阻止( impede的第三人称单数 )

参考例句：

• High packing density greatly impedes the cooling of the array. 高存贮密度很不利于阵列的散热。
• The inflexibility of the country's labor market seriously impedes its economic recovery. 该国劳工市场缺乏灵活性，这严重阻碍了它的经济恢复。

4 necessitates   

使…成为必要，需要( necessitate的第三人称单数 )

参考例句：

• The increase in population necessitates a greater food supply. 人口的增加需要更多食物供应。
• Your proposal necessitates borrowing money. 你的提议使借款成为必要。

5 derived   

vi.起源；由来；衍生；导出v.得到( derive的过去式和过去分词 )；（从…中）得到获得；源于；（从…中）提取

参考例句：

• Many English words are derived from Latin and Greek. 英语很多词源出于拉丁文和希腊文。 来自《简明英汉词典》
• He derived his enthusiasm for literature from his father. 他对文学的爱好是受他父亲的影响。 来自《简明英汉词典》

6 demonstration   

n.表明，示范，论证，示威

参考例句：

• His new book is a demonstration of his patriotism.他写的新书是他的爱国精神的证明。
• He gave a demonstration of the new technique then and there.他当场表演了这种新的操作方法。

7 syllogism   

n.演绎法，三段论法

参考例句：

• The ramifications or the mystery of a syllogism can become a weariness and a bore.三段论证法的分歧或者神秘会变成一种无聊、一种麻烦。
• The unexpected bursts forth from the syllogism.三段论里常出岔子。