What is the argument from illusion?

It reasons that since things can appear other than they are, what we are immediately aware of must be an appearance rather than the physical object itself; generalised, it concludes that we never directly perceive physical objects but only mind-dependent items such as sense-data.

What is disjunctivism about perception?

It is the view that a genuine perception and a subjectively indistinguishable hallucination do not share a common mental core; they are two fundamentally different kinds of state. This lets the perceiver in the good case be directly related to the world and so positioned to gain knowledge that the hallucinator lacks.

Perception and Perceptual Knowledge

Perception is the most basic source through which the world impresses itself on us, but the epistemology of perception asks how experience can justify belief about the world given that perception can mislead and that hallucination may be subjectively indistinguishable from seeing.

Definition

Perceptual knowledge is knowledge acquired through the senses, and the epistemology of perception studies how perceptual experience confers justification on beliefs about the external world and what the nature of that experience must be for it to do so.

Scope

This topic covers perception as a source of knowledge and justification: the argument from illusion and hallucination, the classical sense-datum and adverbial theories, intentionalist and disjunctivist accounts of experience, and the question whether perceptual justification is immediate or mediated by beliefs. It connects to foundationalism, which often treats perceptual beliefs as basic, while leaving the structure of justification itself to a neighbouring area.

Core questions

How does sensory experience justify beliefs about external objects?
What is the immediate object of perception — the world, sense-data, or an intentional content?
Does the argument from illusion show that we never directly perceive physical objects?
Is veridical perception epistemically the same as an indistinguishable hallucination?

Key theories

Sense-datum theory: On Russell's view, what we are immediately aware of in perception are mind-dependent sense-data, and knowledge of physical objects is inferred from them, motivated by the argument from illusion.
Disjunctivism: McDowell and others hold that a veridical perception and a matching hallucination are fundamentally different mental states, so the good case can put the world directly within reach of thought and provide knowledge in a way the bad case cannot.

History

Early modern theories of ideas and the empiricist tradition treated perception as the foundation of knowledge while wrestling with illusion. Russell's 1912 sense-datum account formalised the idea that we are immediately aware only of appearances. Reacting against the resulting gap between mind and world, McDowell's 1994 disjunctivism and the broader naive-realist movement argued that veridical experience can directly disclose the world.

Debates

Direct realism versus the argument from illusion: The argument from illusion and hallucination is used to deny that we directly perceive physical objects, supporting sense-datum and representationalist theories; disjunctivists resist by denying that the deceptive and veridical cases share a common element, preserving direct contact with the world in the good case.

Key figures

Bertrand Russell
John McDowell
Robert Audi

Seminal works

russell1912
mcdowell1994

Frequently asked questions

What is the argument from illusion?: It reasons that since things can appear other than they are, what we are immediately aware of must be an appearance rather than the physical object itself; generalised, it concludes that we never directly perceive physical objects but only mind-dependent items such as sense-data.
What is disjunctivism about perception?: It is the view that a genuine perception and a subjectively indistinguishable hallucination do not share a common mental core; they are two fundamentally different kinds of state. This lets the perceiver in the good case be directly related to the world and so positioned to gain knowledge that the hallucinator lacks.