One common argument against substance (mind-body) dualism runs as follows. We know that consciousness is dependent on the brain, because when the brain is damaged it adversely affects consciousness and mental function. (You can prove this point to yourself experimentally by hitting yourself hard on the head with a brick.) Furthermore, it is argued, when brain function ceases altogether, consciousness disappears. (Don’t try to prove this latter point to yourself experimentally; just take it on trust.) Therefore, contra substance dualism, the mind — if it’s a real entity at all — must be ontologically dependent on the physical structures of the brain. We should be physicalists of some kind.
I come across this argument all the time in the writings of naturalists, but it strikes me as a blatant non sequitur. At most it shows that there’s a causal relationship between the mind and the body, which substance dualists insist upon anyway. (The so-called “interaction problem,” which is concerned with how there can be causation between physical and non-physical substances, is a different challenge to dualism, one I don’t propose to address here.) The fact that increasing damage to the brain leads to increasing mental impairment doesn’t at all imply that the mind cannot exist apart from the brain.
Here’s an analogy to elucidate why that’s so. Imagine a spaceship of the kind familiar from sci-fi movies. In this spaceship, the cockpit doubles up as an escape pod. In normal operation, the cockpit is attached to the main ship; whenever the ship moves, the cockpit moves with it, just as it should. If the ship is attacked with (say) photon torpedoes, the cockpit is buffeted about along with the rest of the spacecraft. When the ship is damaged, all of its systems can be affected; thus the operation of the cockpit can be impaired by damage to the ship in which it is housed.
If the ship becomes so badly damaged that it can’t move at all, the cockpit is stuck along with it, since it’s fixed to the ship. But if the spaceship is completely blown apart, the cockpit functions as an escape pod: it can detach from the doomed ship, and once detached, it can move freely again. (In line with a Christian eschatology, we could even extend the analogy such that if the parts of the ship are recovered and reassembled, the cockpit can be reattached — but that’s not necessary for the point I’m making here.)
Alternatively, think of Marvel’s Tony Stark in one of his Iron Man suits. When the bad guy hits Iron Man, Tony gets shaken about. If the suit is damaged, Tony’s ability to operate becomes impaired, and if the suit is damaged to the point where it no longer functions, Tony is grounded — until, that is, he disengages from the suit, at which point he’s able to move freely again. His ‘operation’ is no longer inhibited by the damaged suit.
The point is this: in both cases there are two distinct things that under normal operation are causally connected in such a way that damage to one impairs the function of the other, and yet the second thing can still exist independently of the first if the latter is irreparably damaged or destroyed.
The physicalist may be tempted to reply, “But both of your analogies involve two physical things that are physically connected!” Right, that’s why we call them analogies. The objection misses the point. The analogy is that we have two causally connected things, where neither depends on the other for its existence. On the substance dualist view, the body and the mind are causally connected; that’s a part of the dualist package that can be taken for granted in the defense I’m offering here. My point is simply this: the phenomenon of mental impairment as a consequence of brain damage doesn’t vindicate physicalism over against substance dualism.