It has appeared that, if we take any common object of the sort that is supposed to be known by the senses, what the senses immediately tell us is not the truth about the object as it is apart from us, but only the truth about certain sense-data which, so far as we can see, depend upon relations between us and the object. Thus what we directly see and feel is merely “appearance”, which we believe to be a sign of some “reality” behind. But if the reality is not what appears, have we any means of knowing whether there is any reality at all? And if so, have we any means of finding out what it is like?
Chapter 1, The Problems Of Philosophy, by Russel Bertrand.