On a straightforward view, we directly perceive the world as it is. The way that things look, feel, smell, taste, and sound is the way that they are. We see colours, for example, because the world is coloured. This view of perception is called, somewhat dismissively, naive realism.

Plausibly, perception is a lot more complicated than this. Though things may appear to be coloured to us, our experiences of colour are merely representative of the surface properties of objects; the physical property of reflecting certain wavelengths of light and the colour red as we experience it are two quite different things.

This has led to representative realism, which suggests that perception is not the passive process that the naive realist suggests, that we do not simply receive information about the world through our senses. Rather, we are actively involved in perception, supplying much of the content of our experiences, and must bear this in mind if we are to know what the world is really like in itself.

More extreme than either naive or representative realism is idealism. Idealists, persuaded by the thought that we have direct access only to our experiences of the world, and not to the world itself, have questioned whether there is anything beyond our experiences. A more recent theory that bears some similarities to idealism has also been proposed: phenomenalism.