Phenomenalism is the position that the physical world exists only as sensory data in the perception of minds, and not as a substance or a thing in itself. Phenomenalism is a form of idealism.
Weaker phenomenalism states only that the sense perception can be known to exist, and that it is either meaningless or useless to talk of objects outside of perception. Stronger versions deny that anything exists outside of sense perception.
As a position, Phenomenalism may have began with the immaterialism of Charles Berkeley, who argued that we cannot know objects outside of our perception of them. Berkeley claimed that objects maintain their existence when no person is perceiving them because God is constantly perceiving all objects.
Some interperetations of quantum mechanics seem to be favour phenomenalism. Quantum mechanics states that particles have a fixed velocity and position, but that the precisce observation of one of these properties makes the other unknown. It also suggests that particles do not assume a specific velocity or position until they are observed. This seems counter-intuitive, but this strange behaviour of matter particles would correspond with phenomenalism, since things would not exist in a definite form until observed. Physicists, of course, have different interperetations (and some definite rejections) of some of the claims of quantum mechanics, so this may not prove to be adequate support for phenomenalism.
See: mind-body problem