What is it to perceive? Consider the claims ‘Jack sees the cat’, ‘Jill hears the bell’; what must be the case if such claims are to be true? The following seems essentially right: if Jack is to count as really seeing a cat, then there must be a cat in front of him, and the presence of the cat must be making a difference to Jack. If things would not look any different to Jack even if the cat were not present, then he can hardly count as seeing the beast in front of him. To see the cat, Jack must be – so to speak – visually locked onto it. Similarly, for it to be the case that Jill hears the bell, then the bell must be causally responsible for her auditory state. If things would not sound any different to Jill even if the bell were left untouched or were completely absent then she cannot count as really hearing it. To hear something involves being auditorily locked onto it.

These remarks give us the beginning of a story about what is involved in genuine cases of perception (as contrasted, perhaps, with cases of mere hallucination or experiences in dreams). If Jack is genuinely to see the cat, the cat must causally affect the way things look to him, i.e. the cat must cause Jack to have certain visual experiences. Likewise, if Jill is to count as hearing the bell, then the bell must affect the way things sound to her, i.e. it must cause Jill to have certain auditory experiences.