The most influential attacks on Direct Realism fall into a family of objections known collectively as the Argument from Illusion or Problem of Illusion. These begin by considering a case in which, the Argument from Illusion alleges, the true object of perception differs from the physical thing that you naively take yourself to be perceiving. When you direct your eyes towards the straight stick in water, what you see is obviously bent. But the physical stick is not bent. Therefore, what you see, at least immediately, is not the physical stick. Perhaps it is a mental thing, like an idea or sense-datum. From here, the Argument typically employs what I call a spreading principle to deny Direct Realism altogether. The stick case is, in crucial respects, qualitatively similar to other perceptions of physical objects, so surely all other immediate objects of perception are mental too. It concludes that Direct Realism is wrong: human perceivers do not immediately perceive physical things or properties.

The Problem of Secondary Qualities is an Argument from Illusion. It says that secondary qualities (colour, smell, sound, taste and heat) are possessed by objects of perception but not by physical things. You can perceive them, but they only exist in your mind, as mental properties or objects. Then the Problem considers other properties, like length, solidity, shape, visual shape and texture, in comparison with the secondary qualities. It points out a qualitative continuity between the phenomenological and experiential aspects of secondary quality perceptions and perceptions of other qualities. Consider what it's like to run your hand over a glass table or velvet upholstery, to squeeze a marble or lump of wet clay. Think about the visual appearance of an unusually shaped drinking glass. These experiences do not seem radically different from those of colour, smell, sound, taste or heat. Secondary quality perceptions share many features, including epistemic features, with perceptions of other properties. Why think that you can see a book's shape immediately when you know that its colour, which seems to inhabit the limits of its shape, belongs to an idea? Why think that you can feel the surface of your coffee cup directly when you know that its warmth, which seems to be permeating that surface, dwells only in your mind? Better to think that secondary quality perceptions mirror perceptions of other objects and properties.