What is the problem of perception?

What is the distinctively philosophical problem of perception? Here it is argued that it is the conflict between the nature of perceptual experience as it intuitively seems to us, and certain possibilities which are implicit in the very idea of experience: possibilities of illusion and hallucination. Perceptual experience seems to us to be a relation to its objects, a kind of 'openness to the world' which involves direct awareness of existing objects and their properties. But if one can have an experience of the same kind without the object being there - a hallucination of an object - then it seems that perceptual experience cannot essentially be such a relation. This is the fundamentally philosophical problem of perception; the various philosophical theories of perception in the 20^th and 21^st centuries can be seen as responses to it.