The philosophical distinction between sense and reference, which seems straightforward with a small handful of examples like "Cicero" and "Tully" or "The Morning Star' and "the Evening Star" is what underlies the idea that two domains of discourse can describe exactly the same entity. But too much focus on those examples have made it misleadingly easy to assume that the same individual described under two different descriptions would always have identical causal powers. From this it has been easy to conclude that describing a brain as a group of molecules would take nothing away from its causal powers, and that describing a mind as a brain would take nothing away from its causal powers. When we move away from the paradigm cases of Cicero and Venus, however, this is no longer obvious. Consider the following dialogue:
A: Is it true that Socrates' death was caused by drinking a cup of hemlock?Here’s one way of describing what’s wrong with this conclusion. Each of these different descriptions presupposes a different causal nexus that makes it happen. Consequently, certain attributes will be genuinely causal under one description of an event, and only epiphenomenal under another description. Epiphenomenal properties are just “along for the ride” and have no causal powers of their own. Under the description "Socrates' death" the fact that Socrates is married to Xantippe is epiphenomenal. Under the description, "Xantippe's becoming a widow", the fact that Socrates is married to Xantippe is causal. Thus under the first description, we can have a causal impact on the event only by saving Socrates' life. Under the second description we can have a causal impact on the event by either saving his life or having him divorce Xantippe.
A: Is it true that Xantippe's becoming a widow was also caused by his drinking the hemlock?
B: That is also true.
A: Why were both of these events caused by drinking the hemlock?
B: Because they were the same event, so talking about "both events" is not really correct.
A: Consider a different example: the cup's being empty and Socrates' death were both caused by his drinking the hemlock, yet those are two separate events. That is a very different case from Socrates' death and Xantippe's becoming a widow, is it not?
B: Without a doubt. Socrates' death and Xantippe's becoming a widow are really two different descriptions of the same event, not two different events.
A: Excellent. We must therefore conclude that we could have saved Socrates' life by having him divorce Xantippe.
Physical descriptions and mental descriptions outline different nexa of responsibility, and therefore we can never completely substitute one for the other, even when they both refer to the same events. Physical causes are not the only "real" causes, and mental causes are not dismissible as mere epiphenomena. Under physical descriptions, physical attributes are genuinely causal, and mental attributes are epiphenomenal. But under mental descriptions, physical attributes are epiphenomenal and mental attributes are genuinely causal.
Let us assume that P is a neurological event taking place in a brain. Let us replace P with another physical event Q that takes place in a silicon module newly installed in Jones' brain, and which now performs the exact same functional role as did the neurological event P. Because the silicon event Q is functionally identical to the neurological event P, we still get mental state M resulting from Q just as we got it from P. This means that with respect to M's coming into being, the difference between P and Q is epiphenomenal, because it is only physical, and that physical difference has no causal effect on whether M occurs or not. Similarly, if Socrates had taken the hemlock in the public square, rather than in the prison, he would still have died. In the same way, when we change neurological state P to silicon state Q we still get M, and therefore the physical characteristics that differentiate P from Q are epiphenomenal with respect to the mental processes.