A fundamental philosophical topic both for its central place in any theory of knowledge, and its central place in any theory of consciousness. Philosophy in this area is constrained by a number of properties that we believe to hold of perception.

1. (i) It gives us knowledge of the world around us.

2. (ii) We are conscious of that world by being aware of ‘sensible qualities’: colours, sounds, tastes, smells, felt warmth, and the shapes and positions of objects in the environment.

3. (iii) Such consciousness is effected through highly complex information channels, such as the output of the three different types of colour-sensitive cells in the eye, or the channels in the ear for interpreting pulses of air pressure as frequencies of sound.

4. (iv) There ensues even more complex neurophysiological coding of that information, and eventually higher-order brain functions bring it about that we interpret the information so received.

Much of this complexity has been revealed by the difficulties of writing programs enabling computers to recognize quite simple aspects of the visual scene. The problem is to avoid thinking of there being a central, ghostly, conscious self, fed information in the same way that a screen is fed information by a remote television camera. Once such a model is in place, experience will seem like a veil getting between us and the world, and the direct objects of perception will seem to be private items in an inner theatre or sensorium. The difficulty of avoiding this model is especially acute when we consider the secondary qualities (see primary/secondary qualities) of colour, sound, tactile feelings, and taste, which can easily seem to have a purely private existence inside the perceiver, like sensations of pain. Calling such supposed items names like sense data or percepts exacerbates the tendency. But once the model is in place, the first property, that perception gives us knowledge of the world around us, is quickly threatened, for there will now seem little connection between these items in immediate experience and any independent reality. Reactions to this problem include scepticism and idealism.

A more hopeful approach is to claim that the complexities of (iii) and (iv) explain how we can have direct acquaintance of the world, rather than suggesting that the acquaintance we do have is at best indirect. It is pointed out that perceptions are not like sensations, precisely because they have a content, or outer-directed nature. To have a perception is to be aware of the world as being such and such a way, rather than to enjoy a mere modification of sensation. But such direct realism has to be sustained in the face of the evident personal (neurophysiological and other) factors determining how we perceive. One approach is to ask why it is useful to be conscious of what we perceive, when other aspects of our functioning work with information determining responses without any conscious awareness or intervention. A solution to this problem would offer the hope of making consciousness part of the natural world, rather than a strange optional extra. See also observation, myth of the given.