How did sense-data theories try to explain perception, illusions, and hallucinations?

In analytic philosophy, sense-data theory posits that the immediate objects of perception are not physical objects themselves, but "sense-data"—private, mental entities that represent the world to the observer.12

Explaining Perception, Illusions, and Hallucinations

Sense-data theorists argue that we are always directly aware of sense-data, regardless of whether our perception is normal or abnormal. This framework attempts to unify these experiences:1

• Normal Perception: When viewing a round penny from an angle, it appears elliptical. Theorists argue that we are directly apprehending an "elliptical sense-datum," even though the physical object remains round.1
• Illusions: These are explained as cases where the sense-datum does not accurately represent the physical object. For example, a stick appearing bent in water is a visual sense-datum that deviates from the object's actual state.2
• Hallucinations: In cases like seeing pink rats where none exist, the observer is still experiencing a "sense-datum"—a visual sensation of rats—despite the absence of a corresponding physical object.1

Because sense-data are considered private and mind-dependent, a central challenge for this theory is justifying how we can know that physical objects exist independently of these sensations.12