Illusion vs Hallucination vs Pseudohallucination

All three involve a perceptual experience without an accurate corresponding external reality, but they split apart on three questions: was there a real stimulus to begin with, where does the experience seem to be located, and does the person believe it is real. Getting these three questions in the right order resolves almost every confusion between the three.

Distinguishing table

Dimension	Illusion	Hallucination	Pseudohallucination
Is there a real external stimulus present?	Yes — a real object or sound is present but misperceived	No — there is no external stimulus at all	No — there is no external stimulus at all
Where is the experience located?	In external space, on the real object that was misperceived	In external, objective space, indistinguishable in location from a real perception	In inner, subjective space ("in the mind's eye"), not projected into the external world
Sensory quality, compared to a real perception	Full sensory vividness, since a real stimulus underlies it	Full sensory vividness and clarity, indistinguishable from a real perception	Reduced vividness and substance; typically described as less solid or less real than genuine perception
Does the person believe it is real?	Usually yes, until corrected, since a real stimulus is genuinely present	Usually yes — experienced with full conviction as a real perception	Usually no — the person retains insight that it is an image or experience of their own mind, not an external event
Typical context	Reduced attention, poor lighting, high emotional arousal, or mild sensory impairment, in a person who may otherwise be well	Psychotic illness, organic brain disease, or severe sensory deprivation	Bereavement, high emotional stress, or as a normal variant, often in someone without significant mental illness

Clarifying questions

Was there actually something there that you looked at or heard, or did the experience come from nowhere?
Separates illusion (something was there) from hallucination and pseudohallucination (nothing was there).
Does it feel like it's happening out here in the room with you, or more like it's happening in your mind's eye?
Separates hallucination (external space) from pseudohallucination (inner, subjective space).
At the time, did it feel completely real, the same as anything else you were seeing or hearing — or did some part of you know it wasn't quite real?
Separates hallucination (full conviction) from pseudohallucination (retained insight).

Common student confusion

Students often assume pseudohallucination just means 'a hallucination the person doubts,' but the defining feature is not doubt — it's location. A pseudohallucination is placed in inner, subjective space from the start, not projected outward and then second-guessed. A hallucination can also be held with less than full conviction (some hallucinating patients express some doubt) without that alone making it a pseudohallucination.

Read each in full

Illusion · Hallucination · Pseudohallucination

References

Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 3, 'Disorders of Perception'.