Argument from Hallucination

Overview and Historical Background

The Argument from Hallucination is a central argument in the philosophy of perception, used mainly to challenge direct realism (or naïve realism) and to motivate sense-data or indirect realist theories of perception. It focuses on hallucinatory experiences—cases in which a subject seems to perceive an object or scene, even though no appropriate external object is present.

Although elements of the argument can be traced back to early modern philosophers such as Descartes, Locke, and Hume, it received its most systematic formulation in early 20th‑century analytic philosophy. Figures such as G. E. Moore, Bertrand Russell, and especially H. H. Price developed the idea that in both veridical perception and hallucination we are directly aware of sense-data: non-physical, mind-dependent items such as colored patches, shapes, or visual fields.

The argument is often discussed alongside the Argument from Illusion, which appeals to misperceptions (such as a straight stick looking bent in water). Whereas illusions involve distortion of how a real object appears, hallucinations arguably involve no corresponding object at all. This stronger case is supposed to show that perceptual experience does not put us in direct contact with mind-independent objects.

Core Argument and Its Philosophical Significance

The Argument from Hallucination typically begins with subjective indistinguishability. Consider two experiences:

• A veridical perception: you see a red tomato on a table under normal conditions.
• A hallucination: you ingest a hallucinogen and have an experience that, from the inside, seems exactly like seeing a red tomato in front of you, though there is no tomato.

Proponents claim that the two experiences can be phenomenally identical—they feel and appear the same to the subject. From this they infer that, in both cases, the subject is aware of entities of the same fundamental kind.

The crucial move is the claim that in the hallucinatory case there is no external tomato or appropriate mind-independent object corresponding to the experience. Still, the subject is aware of something red, round, and tomato-like. Since this “object of awareness” depends entirely on the subject’s mental state and does not exist independently in the world, it is said to be mind-dependent. These entities are often called sense-data.

The argument then generalizes to veridical perception. If hallucinatory and veridical experiences can be subjectively indistinguishable and involve the same kind of immediate object, then veridical perception also involves awareness of mind-dependent sense-data. The physical tomato, on this view, is not what is directly perceived. Instead, the subject directly perceives a sense-datum that is caused by, or systematically correlated with, the external object. This is a form of indirect realism or representative realism.

Philosophically, the Argument from Hallucination raises fundamental questions about:

• The nature of perceptual experience: Are we immediately aware of external objects, internal mental items, or something else?
• The epistemology of perception: If perception involves sense-data, how do we know anything about the external world beyond these mental items?
• The metaphysics of mind and world: How are mind-dependent sense-data related to mind-independent objects, if at all?

Major Responses and Debates

The Argument from Hallucination remains highly controversial. Much of contemporary philosophy of perception consists of responses to it and to the sense-data theory it supports. Several major strategies have emerged.

1. Sense-Data and Indirect Realism

Sense-data theorists accept the argument’s conclusion. They hold that:

• In both hallucinations and veridical perceptions, the immediate object of awareness is a sense-datum.
• Sense-data are mind-dependent, private, and possess the properties they appear to have (for example, a red patch is genuinely red).
• External, mind-independent objects are at best indirectly perceived via their causal role in producing sense-data.

This view was influential in early analytic philosophy but has since declined, partly due to worries about the ontological status of sense-data and the alleged “veil of perception” that seems to separate us from the external world.

2. Disjunctivism

Disjunctivists, often defenders of naïve realism, reject the key claim that veridical and hallucinatory experiences are of the same fundamental kind. They argue instead that the experiences are disjunctive:

• In veridical perception, one is directly aware of mind-independent objects, and the experience’s character partly depends on those objects.
• In hallucination, there is no such object, and the experience is of a distinct kind, often characterized negatively (e.g., as not a perception of external objects) or in terms of its causal origin (brain activity, malfunction, etc.).

On this view, subjective indistinguishability does not entail that the experiences share a common, fundamental mental factor. Disjunctivists deny premise (3) of the canonical argument: similar appearance does not guarantee a shared immediate object.

3. Intentional and Representational Theories

Intentionalists or representationalists argue that perceptual experiences are fundamentally representational states—they represent the world as being a certain way. Hallucinations and veridical perceptions can share the same content (e.g., “there is a red tomato on the table”), even if only the latter is accurate.

On many intentional theories:

• There is no need to posit sense-data as objects of awareness.
• What is directly given is content, not a mental object.
• The difference between veridical perception and hallucination is whether the content is satisfied by the world.

These views often grant some common factor between hallucination and perception (they can have the same representational content) but deny that this factor is a mind-dependent object with the properties perceived.

4. Phenomenal Externalism and Naïve Realism

Some contemporary naïve realists and phenomenal externalists fully reject the idea that hallucinations and veridical perceptions share a common core that can be characterized independently of the world. They maintain:

• The phenomenal character of veridical perception constitutively depends on the external objects and their properties.
• Hallucinations are defective or degenerate states that merely mimic, but do not replicate, the structure of genuine perception.
• The appeal of the Argument from Hallucination rests on overestimating the probative force of first-person indistinguishability.

From this standpoint, the argument is seen as trading on an overly internalist picture of experience.

5. Hybrid and “Common Factor” Views

Some philosophers adopt a hybrid position, accepting that there is a common mental factor between hallucinations and veridical perceptions, while also building in dependence on the external world for fully veridical experience. For example:

• There may be a shared non-factive experiential state, but only in veridical cases is this state appropriately caused and sustained by external objects.
• Alternatively, the same type of representational state can be realized in different causal contexts, producing veridical perception or hallucination.

These views attempt to respect the intuitive force of the Argument from Hallucination without endorsing sense-data or fully abandoning direct realism.

The Argument from Hallucination thus continues to play a pivotal role in debates about how perception relates mind to world. Its premises and inferences remain contested, and its assessment often depends on broader commitments about mental content, the nature of consciousness, and the epistemology of experience.