 Collier’s most purely epistemological argument is that we are all familiar with (visual) experiences that are assumed to be of external objects but which do not differ from similar experiences which are clearly not of external objects, yet that there is no discernible difference between the latter and the former, thus that if the latter are in or entirely dependent upon the mind, then so must be the former. He uses examples such as those of imaginary beings, chimeras or centaurs (1713 [1909: 17]), which he supposes we (visually) represent just as vividly (his term, anticipating Hume) as other objects, secondary qualities (1713 [1909: 21–2]), cases of double vision, cases of experiences which change, such as different phases of the moon, when no one would believe that the external objects is changing (1713 [1909: 33]), and mirror images, which everyone believes exist in the mind, not in the piece of glass outside us, and yet are indiscernible from other images of their objects. He also equates being visible with being “present to the mind”, and asks how something could be present to the mind if it were elsewhere from the mind? (1713 [1909: 35]). He infers that to be visible is to be in or dependent upon the mind, and thus that to be outside of the mind would necessarily be to be invisible (1713 [1909: 56]).