The author concludes that either (1) Caravaggio is not an early practitioner of the Baroque style, or (2) we should reject Mather’s definition requiring “Baroque” painting to display opulence, heroic sweep, and extravagance.

This is based on the fact that Caravaggio’s realism and use of light and shadow broke with contemporary styles and influenced Baroque painting.

The conclusion asserts at least one of these must be true: (1) Caravaggio’s paintings were not Baroque, or (2) Baroque paintings do not require the combination of opulance, heroic, sweep and extravagance.

To prove that at least one of these must be true, we must show that if (1) is NOT true, then (2) must be true. Or, that if (2) is NOT true, then (1) must be true.

Let’s say that (1) is NOT true — in other words, that Caravaggio’s paintings WERE Baroque. On the current premises, would we be allowed to infer that Mather’s definition of Baroque painting is wrong? Not necessarily — Mather says that Baroque paintings must have opulence, heroic sweep, and extravagance. Who’s to say that Caravaggio’s paintings don’t have these qualities? The author is assuming that Caravaggio’s paintings don’t have these qualities. This is why the author believes that if we accept Caravaggio’s paintings as Baroque, then Mather’s definition of Baroque must be wrong.