second part of method consists in the knowledge of the conditions
of good definition, and the means of finding them. (4) I will
first treat of the conditions of definition.

[95] (1) A definition, if it is to be called perfect, must
explain the inmost essence of a thing, and must take care not
to substitute for this any of its properties. (2) In order
to illustrate my meaning, without taking an example which
would seem to show a desire to expose other people's errors,
I will choose the case of something abstract, the definition
of which is of little moment. (95:3) Such is a circle. (4) If
a circle be defined as a figure, such that all straight lines
drawn from the center to the circumference are equal, every
one can see that such a definition does not in the least
explain the essence of a circle, but solely one of its
properties. (5) Though, as I have said, this is of no
importance in the case of figures and other abstractions,
it is of great importance in the case of physical beings
and realities: for the properties of things are not understood
so long as their essences are unknown. (6) If the latter be
passed over, there is necessarily a perversion of the
succession of ideas which should reflect the succession of
nature, and we go far astray from our object.

[96] In order to be free from this fault, the following rules
should be observed in definition:-
I. (1) If the thing in question be created, the definition
must (as we have said) comprehend the proximate cause.
(2) For instance, a circle should, according to this rule,
be defined as follows: the figure described by any line