They also belong to a distinct kind of substance—immaterial substance—a kind of ghost-stuff or ectoplasm. Strictly speaking if I say, ‘I’ thought of the Queen and I saluted,’ there is a kind of ambiguity: the ‘I’ that is the subject of the thought is not the ‘I’, the body, that salutes. Thoughts and experiences are modifications in one kind of stuff; movement and position belongs to the other. This part of Descartes’s doctrine marks him as a ‘substance dualist’. It is not just that there are two kinds of properties (mental properties and physical properties) and that persons can have both. It
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