Although gamma is the third letter of the ancient Greek alphabet, the fourth book of Aristotle’s Metaphysics is called Metaphysics Gamma because there are two extant Metaphysics Alphas and (may we suppose) no one could bear to call one of them lesser, so references to the fourth book are given as Metaphysics Gamma (IV) or sometimes Metaphysics IV (3), this being the book where Aristotle outlines three versions of his famous ‘principle of non-contradiction’.