The first theory in the philosophy of mind is dualism, which in basic terms, is the belief that mind is a different substance to matter. Dualists use Leibniz's law, if A = B then A must have the same properties as B, to argue that because matter is subject to the physical sciences while mind is not, then they must be different stuff's. Mind and matter are also different in other ways. Firstly we can, to a point, locate a piece of matter in time and space and observe that piece of matter. But mind is totally different, you can't locate a though and it's generally believed that the mind is private and can't be observed. So you can easily conclude that mind is different to matter because they have different properties, A doesn't equal B. And because mind is some kind of substance, it can't be nothing, it only fits that there is two kinds of substances, mind and matter.
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