Thus, e.g. (1) the equality of its angles to two
right angles is not a commensurately universal attribute of figure.
For though it is possible to show that a figure has its angles equal
to two right angles, this attribute cannot be demonstrated of any
figure selected at haphazard, nor in demonstrating does one take a
figure at random-a square is a figure but its angles are not equal
to two right angles. On the other hand, any isosceles triangle has its
angles equal to two right angles, yet isosceles triangle is not the
primary subject of this attribute but triangle is prior. So whatever
can be shown to have its angles equal to two right angles, or to
possess any other attribute, in any random instance of itself and
primarily-that is the first subject to which the predicate in question
belongs commensurately and universally, and the demonstration, in
the essential sense, of any predicate is the proof of it as
belonging to this first subject commensurately and universally:
while the proof of it as belonging to the other subjects to which it
attaches is demonstration only in a secondary and unessential sense.
Nor again (2) is equality to two right angles a commensurately
universal attribute of isosceles; it is of wider application.
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