"The clearest form of the argument is given by Aristotle. In the fourth
book of the *Metaphysics,"
Aristotle advances two decisive principles regarding primary substance
(*ousia*): (i) necessarily,
for every attribute, a substance either possesses that attribute or it does
not, which is Aristotle's
version of the principle of excluded middle; and (ii) for any substance, if
anything may be
predicted of it then, necessarily, its attributes cannot be accidents only, or
only apparent
properties, the violation of which Aristotle takes to entail contradiction.
Protagoras, apparently,
violates both -- which shows at the least that relativism was thought in the
ancient world to
involve a restriction on, or abandonment of, the principle of excluded middle.
Now,*if* it is not true that reality is changeless, then, of course, (ii)
must be given up; and
if (ii) is abandoned, then, on Aristotle's own reading of (ii), (i) must be
given up also. But the
ancients understood the doctrine, "man is the measure," to entail at least that
reality is not
changeless -- also, therefore, that if man can rightly claim to have knowledge,
than, on Protagoras'
argument, knowledge cannot be addressed to what is changeless in reality. This
much at least
yields a stalemate between Aristotle and Protagoras: thus far, neither one's
thesis is obviously
incoherent. But even this much favors Protagoras, because Aristotle holds that
the violation
of (i) and (ii) yields contradiction. More would need to be said.
Aristotle does have more to say. There is another argument, a bridge
argument, that is decisive
for Aristotle: "if not all things are relative, but some are self-existent,
not everything that appears
will be true"; and *that*, which is tantamount to (ii), must, *somewhere* in
Protagoras' argument,
yield the denial of those properties of particular substances *that are
changeless.* Nothing could
be more reasonable. The only trouble is that Protagoras rejects the thesis
that there *is* something
changeless, and Aristotle nowhere shows convincingly that *that* produces
contradiction, except,
trivially, *by* presupposing the truth of what must first be shown to be true.
So Aristotle fails.
Certainly, in our own time, nearly every prominent thinker either believes that
reality is not
changeless or believes that it is not demonstrably true that believing *that*
cannot but be
incoherent."
(Margolis, Joseph, 'The Truth About Relativism' (Paperback), pp.77-78)
_______________________________________________________________________
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