It seemed obvious to David Hume (1758) that the objects we perceive visually get smaller as we move away from them, yet the objects themselves do not change. Therefore the perceptions we experience must be of the mind, just copies of reality (these are called sense-data). In the case of a hallucinations the sense-datum exists without the physical object. In an illusion the sense-datum is distorted.
An alternative view, Naïve realism, tells us that it is the real world that we are visually experiencing in the normal case but this requires a separate explanation for the cases of illusion and hallucination.
We will be looking for some explanations of these phenomena with some help from neuroscience.