How should we characterize the nature of perceptual experience? Some theorists claim that colour experiences, to take an example of perceptual experiences, have both intentional properties and properties called 'colour qualia', namely, mental qualitative properties which are what it is like to be conscious of colour. Since proponents of colour qualia hold that these mental properties cannot be explained in terms of causal relations, this position is in opposition to a functionalist characterization of colour experience.

Suppose someone hears a loud noise and at the same time sees a yellow flash. It seems hard to deny that the person can experience loudness and yellowness together. However, since loudness is experienced by the auditory sense whereas yellowness is experienced by the visual sense it also seems hard to explain how.