Continuing the "mad scientist switching brains" thought experiment, supposedly the switch is not permanent, and both persons know it. Which body would one empathise with more? I think the answer would depend on how long one is to live in one versus in the other. If the switch is for only 10 minutes, it is minor. On the other hand, if the switch is for almost one's entire remaining life, it's a different matter. Anyway, philosophers have macabre imaginations.
The muslim saint Rumi (1207-1273) discussed this more than 4 centuries before Locke, and used it to bring advantage to people, see for example Mathnawi, book V, verses 1772 onwards (e.g. "... At daybreak the soul recognises its own body and re-enters its own ruin (like treasures hidden in waste places). It recognises its own body and goes into it: how should the soul of the goldsmith go to the taylor? The soul of the scholar runs to the scholar, the spirit of the tyrant runs to the tyrant...")
Two logical proofs of nonmaterial substance: 1) There is an identity of every living being that is constant throughout their entire life. The mind and body change radically throughout the life. Therefore, there must be a nonmaterial substance which is the identity of life, known as the soul. 2) There is something which gives life to the body. The only difference between a living and dead body is the presence of consciousness, and no material adjustment can revive life. Therefore, that which gives life to the body is a nonmaterial substance, known as the soul.
