If you believe in idealism, you are disbeliving in what?
An argument for idealism, such as those of Hegel and Berkeley, is ipso facto an argument against materialism. Matter can be argued to be redundant, as in bundle theory, and mind-independent properties can in turn be reduced to subjective percepts. Berkeley presents an example of the latter by pointing out that it is impossible to gather direct evidence of matter, as there is no direct experience of matter; all that is experienced is perception, whether internal or external. As such, the existence of matter can only be assumed from the apparent (perceived) stability of perceptions; it finds absolutely no evidence in direct experience.