In philosophy, idealism is the group of metaphysical philosophies that assert that reality, or reality as humans can know it, is fundamentally mental, mentally constructed, or otherwise immaterial. Epistemologically, idealism manifests as a skepticism about the possibility of knowing any mind-independent thing. In contrast to materialism, idealism asserts the primacy of consciousness as the origin and prerequisite of material phenomena. According to this view, consciousness exists before and is the pre-condition of material existence. Consciousness creates and determines the material and not vice versa. Idealism believes consciousness and mind to be the origin of the material world and aims to explain the existing world according to these principles.
(Wikipedia entry for Idealism)
The problem begins with metaphysics. Is it possible to answer objectively the question of the nature of reality? Of course, knowledge of reality must necessarily begin with the mind perceiving. But this does not logically lead to the idea that only mind is necessary. A sensed reality is the only reality we have. The mind of a person is seen not directly, but always through her or his material. Idealism is correct in that knowing is a mental process, and that knowing does not mean access to the thing. But this does not mean primacy of the mind. The mind cannot exist without the body. Material determines the consciousness, not vice versa. Our knowledge of the science of neurology and the brain should be enough evidence for us to dismiss Idealism as a viable philosophical stance.