Sensory experience

In the philosophy of perception, the theory of sense data was a popular view held in the early 20th century by philosophers such as Bertrand Russell, C. D. Broad, H. H. Price, A.J. Ayer, and G.E. Moore. Sense data are supposedly mind-dependent objects whose existence and properties are known directly to us in perception. They are supposed to be unanalyzed experiences inside the mind, which appear to subsequent more advanced mental operations exactly as they are.

Sense data are often placed in a time and/or causality series such that they occur after the potential unreliability of our perceptual systems, but before the possibility of errors during higher-level conceptual analysis, so are incorrigible. They are thus distinct from the 'real' objects in the world outside the mind, about whose existence and properties we often can be mistaken.

Talk of sense-data has since been largely replaced by talk of the closely related qualia. The formulation the given is also closely related. None of these terms has a single coherent and widely agreed-upon definition, so their exact relationships are unclear. One of the greatest troubling aspects to 20th century theories of sense data is its unclear rubric nature.

Bertrand Russell heard the sound of his knuckles rapping his writing table, felt the table's hardness and saw its apparent colour (which he knew 'really' to be the brown of wood) change significantly under shifting lighting conditions.

H. H. Price found that although he was able to doubt the presence of a tomato before him, he was unable to doubt the existence of his red, round and 'somewhat bulgy' sense-datum and his consciousness of this sense-datum.

When we twist a coin it 'appears' to us as elliptical. This elliptical 'appearance' cannot be identical with the coin (for the coin is perfectly round), and is therefore a sense datum, which somehow represents the round coin to us.

Consider a reflection which appears to us in a mirror. There is nothing corresponding to the reflection in the world external to the mind (for our reflection appears to us as the image of a human being apparently located inside a wall, or a wardrobe). The appearance is therefore a mental object, a sense datum.

The idea that our perceptions are based on sense data is supported by a number of arguments. The first is popularly known as the Argument From Illusion. From a subjective experience of perceiving something, it is theoretically impossible to distinguish perceiving something which exists independently of oneself from an hallucination or mirage. Thus, we do not have any direct access to the outside world that would allow us to reliably distinguish it from an illusion that caused identical experiences. Since (the argument claims) we must have direct access to some specific experiential entity in order to have the percepts that we do, and since this entity is not identical to the real object itself, there must be some sort of internal mental entity somehow correlated to the real world, about which we afterwards have perceptions, make judgments, etc. This entity is a sense-datum.

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