The impossibility of actually seeing a nonexistent thing places an important limit on the argument: something that has not come into existence cannot be observed as an existing thing is. Existence and nonexistence therefore occupy a central place in the discussion of what sight can apprehend.
Yet the fact that a nonexistent object is not seen does not, by itself, prove that existence is the complete effective cause of visibility. The logical relation among possibility, existence and causal explanation remains subject to further examination.