We ought to pay attention only to the intrinsic properties of a work of art.
Conclusion
What is really aesthetically relevant is not what a painting symbolizes, but what it directly presents to experience.
Evidence
What is really aesthetically relevant is not its extrinsic properties, but its intrinsic properties (i.e. what it directly presents to experience)
Evaluate
With my cleaned up version, where I arrange the Conclusion and Evidence in parallel, it's easier to see that the author seems to be assuming .
How did we get there?
Both the premises and the conclusion are saying,
"Look at this, not that"
intrinsic properties extrinsic properties
what it directly what a painting presents to experience symbolizes
aesthetically relevant irrelevant to our aesthetic interactions
Goal
When we're doing Sufficient Assumption, and there's a New Term in the conclusion (i.e. a term that was never discussed or defined in the Evidence), then that New Term must be in the correct answer, in order for us to get a closed circuit of logic.