Full Idea
Only a causal theory of perception will respect the facts of physiology and physics ...meaning a theory which maintains that for a subject to perceive a physical object the subject should enjoy some appropriate perceptual experience caused by the object.
Gist of Idea
Science requires a causal theory - perception of an object must be an experience caused by the object
Source
E.J. Lowe (Locke on Human Understanding [1995], Ch.3)
Book Reference
Lowe,E.J.: 'Locke on Human Understanding' [Routledge 2004], p.59
A Reaction
If I hallucinate an object, then presumably I am not allowed to say that I 'perceive' it, but that seems to make the causal theory an idle tautology. If we are in virtual reality then there aren't any objects.