The following type of hypothetical proof also begs the question.
If evil is definable as the divisible, and the definition of a thing's
contrary-if it has one the contrary of the thing's definition; then,
if good is the contrary of evil and the indivisible of the
divisible, we conclude that to be good is essentially to be
indivisible. The question is begged because definable form is
assumed as a premiss, and as a premiss which is to prove definable
form. 'But not the same definable form', you may object. That I admit,
for in demonstrations also we premise that 'this' is predicable of
'that'; but in this premiss the term we assert of the minor is neither
the major itself nor a term identical in definition, or convertible,
with the major.
Again, both proof by division and the syllogism just described are
open to the question why man should be animal-biped-terrestrial and
not merely animal and terrestrial, since what they premise does not
ensure that the predicates shall constitute a genuine unity and not
merely belong to a single subject as do musical and grammatical when
predicated of the same man.
7
How then by definition shall we prove substance or essential nature?
We cannot show it as a fresh fact necessarily following from the
assumption of premisses admitted to be facts-the method of
demonstration: we may not proceed as by induction to establish a
universal on the evidence of groups of particulars which offer no
exception, because induction proves not what the essential nature of a
thing is but that it has or has not some attribute.
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