Academic arguments for the indiscernibility thesis

The Academics offered an argument from twins or perceptually indiscernible objects and an argument from dreams or madness in support of the indiscernibility thesis: that every true perceptual impression is such that some false impression just like it is possible. I claim that these arguments, unlike modern sceptical arguments, are supposed to establish mere counterfactual rather than epistemic possibilities. They purport to show that for any true perceptual impression j, there are a number of alternative causal histories j might have had which would not have resulted in any change in the way in which j represents its object