Distinguish clearly these bitter yet fertile human truths, flesh of our flesh, and admit them heroically:
(a) the mind of man can perceive appearances only, and never the essence of things;
(b) and not all appearances but only the appearances of matter;
(c) and more narrowly still: not even these appearances of matter, but only relationships between them;
(d) and these relationships are not real and independent of man, for even these are his creations;
(e) and they are not the only ones humanly possible, but simply the most convenient for his practical and perceptive needs.