 Our discussion of Aristotle ended by returning to the issue of the unity of substantial form—what is the difference between a collection of ‘essential’ properties and the form as a unitary entity. Various aspects of this issue of the unity of substantial form preoccupied medieval philosophers. According to Aquinas, a substance possessed only one form, and its matter was the essentially characterless prime matter. In other words the informed parts of an object—in the case of a living creature, its organs, and the various kinds of stuff that constituted it (in the end, quantities of earth, fire, air, and water) did not possess their own forms, but were informed by the overall substantial form. Others disagreed. Some of the disagreements were internal to the hylomorphic system. Duns Scotus, for example, argued that a dead person’s body was the same body as had existed when that person was alive. The soul had departed, so the form that was the immortal soul could not be identical with the form of the body. This argument may seem to be specialised to the case of the human immortal soul, but, presumably, a similar problem applies to a dead parrot: it has lost its perceptive and vegetative souls, yet is the same body—even if it is no longer a parrot.