In the latter case, the elementary particles are only virtually present, and that virtual existence can be turned into actual existence only by destroying the physical thing in which they are virtually present. A correct understanding of perception shows why such efforts are vain as well as unnecessary. Except for the mental act of perception, all other acts of the mind, such as imagination, memory, and conception, present us with objects concerning which we must ask whether, in addition to being objects of the mind, they also exist in reality. But in the one case of perception, we cannot separate our having the perceptual object before our minds from asserting that it also really exists. If that were not so, there would be no distinction between hallucinating and perceiving.
Hallucination is pathological.
Normal perception is always the perception of something that has existence in reality. If we do not assert that the perceptual object also really exists as a perceptible thing, we cannot say that we are perceiving it. No proof of an external world is necessary. Existence has three modalities. The first is real existence, existence independent of the human mind and unaffected by it.