You’ve missed my point. It’s a trivial truism that the same atoms can be part of different substances at different times. That is not in question. All ordinary matter in the universe is made up of the same limited set of 92 naturally occurring elements (just as a technical aside, and to be absolutely correct, these elements do not “make up everything in the universe”, because they do not include elementary bosons - photons, W and Z bosons, and so on, nor do they fundamentally include leptons, nor do they include any baryons that are composed of other than purely up and down quarks or more than three quarks - such as hyperons, nor do they include mesons, nor do they include whatever constitutes dark matter: There is a lot of stuff in the universe other than matter composed of what you call elements.) Returning to the case in point, not only is all ordinary matter made up of the 92 different elements but identical atoms can be part of different substances at different times. That is not at all controversial. What is in question is whether an entity can change its substance without any perceptible physical change and the evidence is that it cannot. Take the very example that you used of water - it is made up of two hydrogen and one oxygen atoms, but the substance of water is not a simple blend of the substances of hydrogen and oxygen. Irt is something entirely different. Molecular hydrogen and biatomic molecular oxygen have entirely different substantial properties from water and from one another, as does triatomic oxygen (ozone) and H2O2 (hydrogen peroxide). This simple case shows that the substantial (and essential) properties of entities are linked semantically to their molecular structure (and emergent properties of matter) rather than to their constituent atoms. Contrary to what you say, the important constituents of bread, and that which makes bread bread rather than an elephant or a dolphin or an oak are molecular rather than atomic, and these do undergo fundamental change on being incorporated into humans. Its molecular structure changes from that of a cereal (processed in a particular way that also is needed to justify the token, bread) to that of a human and the change is sensible and profound. Where can you give an example of an entity which changes its substance without any sensible change - in philosophy, in science and in common semantics, an entity which does not change sensibly from one point in time to another remains substantially the same.
There are many question begging fallacies in the above - To say that what makes you the same person as 40 years ago is that you are that person is tautological. Memories no more presuppose personal identity than bunions do - they are both physical phenomena, and without memory no personal identity could exist. The organism that we refer to as Bob is associated exclusively with the continuous (never discontinuous) physical organism, and the personhood of Bob is encoded in the memories and sense-of-self grounded in the brain of Bob the organism (and never in any other organism’s brain, or stomach or bunions).
On the contrary, it is deeply logically and metaphysically impossible (not to mention biologically impossible) because the substance of human beings is associated exclusively with a single body that is the ground for that person. To say that Bob can remain Bob while inhabiting Alice’s body and having exclusively Alice’s memories (and character and fears and hopes and preferences and opinions - all of which necessarily go with Alice, the body) is to do as much logical and semantic and philosophical violence to the concept of substance, as the claim that the consecrated Host is someone’s total substance, body, blood and soul, does to the concepts of both substance and essence.
Alec
evolutionpages.com
