 In accidental changes there is always a substance to underlie the change, but this is not true for substantial changes, since these involve the coming to be or passing away of a substance (see the amusing remark of Irving Copi, quoted at the start of the entry on  identity over time).   In these cases, the thing that underlies is the matter of the substance. When someone builds a house, it is the bricks which persist through the change. They transition from a state of not being a house to acquire the property of being a house. Aristotle often uses the example of artefacts like houses, even though he does not regard them as substances properly-speaking (Metaphysics vii 17, 1041b28–30), because their matter is more straightforward to identify. Nevertheless, the same analysis holds in the case of organisms, which are the substances proper: when an organism is created or destroyed, when an acorn becomes an oak tree, or a human dies, there must be some matter which persists through the change. To say otherwise would be to say that things can come to be out of, or vanish into, nothing, and Aristotle understandably agrees with his predecessor Parmenides that this is impossible (Physics i 8, 191a23–b17). Aristotle’s metaphysics takes as its starting points observed phenomena, and seeks to preserve common sense beliefs where possible. We never experience anything simply appearing or disappearing at random.