The argument from opposites is a second-order subject argument that supports a value with another value (2 sub VV).
ANALYSIS OF AN EXAMPLE
But since, old man, false statements are persuasive among mortals, you should believe the opposite too: That many truths turn out to be incredible to mortals
The arguer states in the conclusion that something turns out to be incredible and supports this claim by stating that the opposite is persuasive. Since the argument fits the form q is T, because r is T, it can be identified as a second-order subject argument. In this specific case, q is instantiated by ‘that many truths turn out to be incredible to mortals’, T by ‘true’, and r by ‘that false statements are persuasive among mortals’.
That many truths turn out to be incredible to mortals (q) is true (T), because that false statements are persuasive among mortals (r) is true (T)
Second-order subject arguments are further differentiated by identifying the types of statement in the conclusion and the premise. In this case, both are statements of (logical) value, which means that we are dealing with a second-order subject argument supporting a value with a value (2 sub VV).
The trivial name of second-order subject arguments is derived from the characterization of the relationship between the subjects q and r, which are both propositions that themselves consist of a subject and a predicate. In this example, the argument draws on the opposition between the subjects of q and r – many truths and false statements – and that between the predicates of q and r – being incredible and being persuasive. We can therefore call it an ‘argument from opposites’.
The example is mentioned by Aristotle in his description of the topos ‘from opposites’ in Rhetoric 1397a and is taken from Euripides, Thyestes, fragment 396 (see Kennedy, 2006, p. 172).