Most commentators on Baroque painting consider Caravaggio an early practitioner of that style, believing that his realism and novel use of the interplay of light and shadow broke sharply with current styles of Caravaggio's time and significantly influenced seventeenth-century Baroque painting.
Conclusion (therefore)
One must abandon the opinion of this majority of commentators or reject Mather's definition that says Baroque paintings must display opulence, heroic sweep, and extravagance.
Evidence
Most commentators consider Caravaggio an early Baroque artist.
Evaluate
The conclusion is saying, .
Either the commentators' opinion is wrong, or Mather's definition of Baroque is wrong. They're mutually exclusive.
Once we insert the specific ideas into those placeholders, the conclusion becomes,
Apparently, our author doesn't think that Caravaggio's paintings fit Mather's definition of Baroque.
Goal
We need to prove that Caravaggio's paintings don't fit Mather's definition. So we need to know at least one of these things about Caravaggio's paintings: - they don't display opulence - they don't have heroic sweep - they don't have extravagance