Universa Philosophia

Definition, from Aristotle, first of Topics, ch. 4, is oratio significans, quod quid est rei: that is, explicans naturam rei. Oratio is not taken for the vocal utterance alone, for thus the definition would be mutilated: but oratio is taken generally for the act of the intellect signifying the quiddity or nature of a thing by voice or writing. For example, man is composed physically of body and soul: he can be defined, homo constat ex corpore, et anima. He is composed likewise logically of animal and rational, and can thus be defined, homo est animal rationale: where corpus, et anima, animal, et rationale, are the nature showing the quiddity of man, they are not the oratio explaining the nature of the thing, they are the nature of the thing itself explained through the oratio, which nature of the thing explains the defined: that is, they bring it about that the defined is perfectly known, therefore it is objective definition: because in the account of object it has to explain what homo (which is the defined) has confusedly and as it were involvedly.

It is called definitio, not because it puts an end to the object, but because it itself is bound to one object, so as not to wander through others as a confused act wanders: for animal and rationale are so proper to man that a clear cognition of them belongs to man alone, at whom that cognition is terminated. But if you said, homo est animal bipes, man would not be defined: because to have two feet belongs as much to the crane as to man. Wherefore if God cannot be defined, it is not because he lacks an end, but because he cannot be known quidditatively in this life; as neither can the other separate substances. But he can be defined imperfectly: because he has an account somehow common with creatures, and a difference proper to himself.

Aristotle, 3 Metaph., ch. 3, says that all things are known through definition: but that genus is the principle of definition: whence the dialectical adage flowed, Definitio constat genere, et differentia: because it ought to represent the genus and the difference. The proximate genus indeed, in which several species agree, but an essential difference, as animal rationale is the definition of man: because animal is the genus in which homo and leo proximately agree: but rational is the difference by which man essentially differs from them: but if the difference be accidental, it will not properly be a definition, but a description. Logical definition consists of genus and difference; but physical definition consists only of two specific parts: as man of body and rational soul. But body is somehow common also to the lion, yet it is not a genus. All genera can be defined, but individuals not so well: review disp. 10 of logic, sect. 2.

Laws of definition. The first, that definitio, et definitum convertantur: that is, that the consequence from one to the other be valid. The second, that it ought to belong to every defined thing, and not to others: which second rule is contained in the first; an example of both: est homo, therefore est animal rationale: est animal rationale, therefore est homo: and so animal rationale belongs to all men, and not to others. The third rule, a definition is clearer than its defined, proper, not metaphorical: for if anyone, to define sleep, should say, somnus est mortis imago, he has said nothing; which phrases are suited to poets and orators: but to the Dialectician none is suited except a clear and brief one: wherefore Aristotle compared the orator to the palm of the hand; but the Dialectician to the fist: because the latter speaks compressedly, the former diffusely. If anyone asks, which is this definition? Answer Physical, or Logical. But if you are asked of what sort is this definition? Answer good or bad, and do not say true or false: because it can be true and bad, because it is not converted with the defined, as homo est animal bipes is bad and true; but if the definition is good, it is necessarily true.