 In the second half of the 13th century, the debate on essence was focused on the relation between substantial essence (the so called ‘forma totius’, e.g., humanity) and the singular substantial form which, together with the singular matter, brings the substantial composite about (the so-called ‘forma partis’, e.g., human souls, which in union with human bodies give rise to human beings). Medieval authors disputed a lot about whether or not the essence (the forma totius) of a thing had to be somehow identified with the singular substantial form (the forma partis). Following Averroes’ commentary on the Metaphysics (book VII, t.c. 34), some authors maintained that it was so. Others, such as Thomas Aquinas (Summa theologiae, I, q. 75, a. 4), supported the thesis that the substantial essence of a thing and its singular form are somehow distinct entities, since the essence includes the common matter of the species, namely the material proper to all the members of the species connected with the essence (flesh and bones for human beings).