...one cannot prove that physical substance is dubitable, but only one’s perception of physical substance.
Cartesian dualism—the idea that the mind is non-physical and distinct from physical substance—is one of the most influential ideas in the history of philosophy. It is, of course, named after René Descartes, who offered the first arguments for Cartesian dualism in his 1641 treatise, Meditations on First Philosophy. A formulation of one such argument follows:
I. Two identical ‘things’ cannot have different properties.
II. Physical substances lack the property of indubitability.
III. Mental substances have the property of indubitability.
ஃ Mental and physical substances cannot be identical ‘things.’
In other words, one can doubt the reality of physical substances—perhaps we’re merely experiencing a hallucination when we perceive physical objects—but one cannot doubt that their mind exists (I think, therefore I am). Taking dubitability as a property, Cartesian dualism is thereby proven.
However, this argument’s second premise is flawed: one cannot prove that physical substance is dubitable, but only one’s perception of physical substance. Because perception is merely activity within our minds, it can say nothing as to physical substance in itself. The Cartesian dualist might attempt to establish physical substance’s dubitability by responding that our perception is so fallible that we can’t know anything about physical substance whatsoever, but in the face of such a rejoinder, it is enough to point out that knowing nothing about an object is a far cry from knowing that object to be dubitable. As such, one cannot conclude that physical and mental substances have different properties, and Cartesian dualism, on these grounds, fails.