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The divided-brain objection

The unity dualism takes for the mark of a simple substance comes apart when a brain is severed.

Objects to Substance dualism

Parfit · Reasons and Persons, § 87 ("Divided Minds") · 1984

1. The argument rests entirely on one premise: what thinks cannot be parted. If it falls, the distinction between the indivisible mind and the divisible body falls with it.
2. Yet in epileptic patients the bundle joining the two hemispheres has been severed, producing what is called a split brain. They live normally afterwards; but in certain tests one hand points to what the subject declares not to see, and he at once invents a reason for the gesture. One and the same brain then seems to host two streams of experience that ignore each other.
3. The sense of being one therefore does not prove one is dealing with a simple substance: it may be the product of successful communication between parts, and it comes apart when that communication is cut.
4. Long before such operations, mental life showed its seams: what admits of degrees (sleep, drunkenness, dementia), what dissociates, what is built up in stages in a child, resembles an assembly rather than something indivisible, and joins the question of personal identity.

Therefore The mind's indivisibility is not a given: it is an appearance produced by an intact brain. The argument's decisive premise falls.