From Sir Richard Gregory
… all pictures are impossible: they have a double reality. They are seen both as patterns of lines, lying on a flat background and also as objects depicted in a quite different, three-dimensional, space. No actual object can be both two- and three-dimensional and yet pictures come close to it. Viewed as patterns they are seen as two-dimensional; viewed as representing objects they are seen in a quasi three-dimensional space. Pictures lying both in two and in three dimensions are paradoxical visual inputs. Pictures are also ambiguous, for the third dimension is never precisely defined
