Reality and Appearance
The word "reality" has had a special meaning in modern times since Immanuel Kant's so-called Copernican revolution in philosophy declared that the thing in itself the "Ding an sich" is not knowable by us.
It was Kant's Copernican revolution that introduced into philosophy the fundamental error of ontological idealism, an error not found in ancient and medieval thought. Before Kant, there were self-refuting skeptics, but no idealists. The word "reality" signified not only that which exists in complete independence of the human mind, but also that which is knowable by and intelligible to us.
The great philosopher of antiquity and of the Middle ages were all realists. Aristotle and Aquinas, for example, asserted that we could know and understand a past that no longer existed, but was nevertheless an object of perceptual thought. That reality has existence for us, (See Cognition.)
It is in this context that the word "appearance" is used. It has different meaning for realists who naively assert that reality is exactly the same as it appears to be and those who assert more critically that what appears to us may not be identical in character with what really exists.