Direct realism, indirect realism and the arguments from perceptual variation, illusion, hallucination and time-lag, Berkeley's idealism and the master argument, and responses including Locke's primary and secondary qualities and Russell's best-hypothesis argument.

What this dot point is asking

AQA wants you to compare three theories of perception (direct realism, indirect realism and Berkeley's idealism), deploy the arguments from perceptual variation, illusion, hallucination and time-lag, and evaluate the main responses, including Locke's distinction between primary and secondary qualities and Russell's argument that an external world is the best hypothesis.

Direct realism

Its strength is fit with common sense and the simplicity of having no intermediary. The main challenges are the arguments below, which suggest what we are immediately aware of cannot always be the object itself.

Indirect realism

This is Locke's representative theory of perception. It explains illusion and hallucination smoothly but raises the veil of perception worry: if we only ever directly access sense-data, how do we know any external world causes them, or what it is like? Two sceptical consequences follow. First, we cannot be certain the external world exists at all, since the sense-data would be just the same if there were no objects (this is the door through which scepticism and Berkeley's idealism enter). Second, even if it exists we cannot know its nature, because we only ever compare sense-data with other sense-data, never with the object itself. Indirect realism must answer both worries, and Locke and Russell supply the standard replies below.

The arguments against direct realism

• Perceptual variation (Russell's table). A table's apparent colour and shape vary with viewpoint and light, yet the table itself does not change; so the immediate object of perception is the changing sense-datum, not the table.
• The argument from illusion. A straight stick looks bent in water; we are immediately aware of something bent, but the stick is not bent, so we are aware of sense-data.
• The argument from hallucination. A hallucination can be experientially indistinguishable from veridical perception while no object is present; the common factor must be sense-data.
• The time-lag argument. We see the Sun as it was eight minutes ago; we are aware of light-borne sense-data, not the object as it is now.

The direct realist has replies to each. Against perceptual variation, the realist says we directly perceive the real table, which genuinely has different appearances from different points of view; "looking elliptical from here" is a relational property of the round object, not evidence of a sense-datum. Against illusion, the realist distinguishes how things are from how they look: the stick really is straight and merely looks bent, so no bent object or sense-datum need exist. The hardest case is hallucination, because there is no object at all; here disjunctivists (such as those drawing on the work of Snowdon and McDowell) argue that veridical perception and hallucination are different mental states that merely seem the same from the inside, so there is no "common factor" sense-datum shared by both. Whether this is convincing is a standard evaluation point.

Berkeley's idealism

Berkeley also attacks the primary or secondary quality distinction, arguing that all qualities are mind-dependent.

Responses

• Locke: primary and secondary qualities. Primary qualities (extension, shape, motion, number) resemble properties really in the object; secondary qualities (colour, taste, smell) are powers in the object to produce ideas in us. This grounds indirect realism's claim that sense-data represent real features.
• Russell: the external world as the best hypothesis. A mind-independent world is the simplest and best explanation of the order, coherence and involuntariness of our experience, even if it cannot be proved with certainty.
• Against idealism: it seems to collapse into solipsism without God, the reliance on God is contested, and it struggles to distinguish veridical perception from illusion.