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Chapter X: The nature and forms of DefinitionEdit

A definition which gives the real nature of a thing also gives its cause, and thus differs only in form from demonstration. There are three classes of definition: (1) An indemonstrable explanation of the essence of a thing; (2) Definitions which resemble a syllogism concerning a thing’s essence; (3) A conclusion of an essential demonstration.

Since definition is, as we have said, the expression of a thing’s essence, it is clear that one kind of definition may give the meaning of the name, or of some other expression having the force of a name:—e.g. the definition of the meaning of ‘nature of triangularity as such.’ On learning that the thing corresponding to the name exists we enquire why it exists, for it is difficult to grasp the cause of a thing when we do not know previously that the thing exists. The reason of this difficulty has been mentioned before (II, c. 8), namely that we do not know whether a thing exists or not except accidentally.

An expression may have unity in two ways; either from a union of the separate parts (in the manner in which the whole Iliad is a unity), or from predicating an attribute of the subject essentially, not accidentally. Thus one form of definition is that which gives the meaning of a word, as mentioned above. Another explains the reason why a thing is. Hence the former explains signification but proves nothing, while the latter clearly gives a kind of demonstration of the subject’s essence, and differs from demonstration only in form. Thus there is a difference between saying, ‘Why does it thunder?’ and ‘What is thunder?’ To the former one would answer, ‘because fire is quenched in the clouds’; to the question, ‘What is thunder?’ ‘Thunder is the noise of fire being quenched in the clouds.’ Thus the same thought is expressed in two different ways; the former answer containing all the parts of a demonstration, the latter being stated as a definition.

Further there may be a definition of thunder, as ‘a noise in the clouds,’ which forms only the conclusion of the demonstration of the essence of thunder. Lastly the definition of ultimate terms is an indemonstrable proposition stating the essence of the subject. Consequently definition is either (1) An indemonstrable expression of the thing’s essence, or (2) A syllogism expressing its essence, differing only in form from demonstration, or (3) The conclusion of a demonstration which states the subject’s essence. What has been said is enough to shew how far a thing’s essence is demonstrable, how far the reverse; also what things admit of demonstration and what do not, in what senses ‘definition’ is used, and in what ways it does or does not prove a thing’s essence, and in what cases this can be done; lastly the relations of definition to demonstration have been given, and it has been shewn how far the two may have the same object.