The issue: "Does the reality of sensible things [1] consist in being perceived, or [2] it is something distinct from their being perceived, and that bears no relation to mind?" (P 163b)

The idealist, represented by Philonous, says the first: The reality of sensible things consists in their being perceived: for sensible things to be is to be perceived.

The realist, represented by Hylas, says the second: sensible things have "a real absolute being, distinct from and without any relation to their being perceived." (P 164a)