Can we be certain that other people are conscious?

How does an individual know anyone else is conscious at all? How does an individual rule out the possibility that every other person is merely a sophisticated biological robot wholly lacking any experiences?  After all, we never actually see anyone else's consciousness. So, is it at least conceivable that everyone -- apart from you, the reader -- are simply unconscious automatons, or what philosophers refer to as p-zombies? That no-one else, apart from you, has an inner conscious life? 

The answer, of course, is that an individual cannot be absolutely certain that others are conscious. But that it is highly likely since other people act like us. They scream in pain when hurt, display appropriate emotions on their faces under the relevant circumstances, and so on. So one can confidently infer that everyone else is conscious, just like oneself. That's the commonsensical view and certainly the view that I hold.

There is an apparent intractable difficulty here though.  One of the mainstream suppositions of scientists is that the world is causally closed, meaning that every event that ever happens has a full explanation in terms of antecedent material causes. This includes us human beings too, and so includes all the material processes occurring within our brains. But if everything people ever do and say are purely due to material causes occurring in their brains rather than being an expression of their consciousness, then it seems we cannot infer that other people are conscious! 

Let me try to convey this very important point again. Suppose a robot declares it is conscious; that it feels fear, hope and so on. We could check that out by disassembling it.  We will find out that the robot says all these things, not because it is conscious and actually experiencing such emotions, but because it is programmed in such a manner to say these things. That being so, we surely have zero reasons to ascribe consciousness to it. Similarly, given the important proviso of causal closure, the exact same applies to us human beings. We can examine the inside of someone's brain and by noting the material chains of causes and effects, we can, at least in principle, figure out exactly why that person behaves and says what he does. It's all just material causes and effects playing out, and we have no more reason to ascribe consciousness to that human being than we did the robot.

How do those who advocate causal closure escape this absurdity?  How can anyone who subscribes to causal closure believe that anyone else is conscious at all given that everyone's behaviour is just the result of material causes playing out?  It seems we have no more reason to ascribe consciousness to anyone else than we have reason to ascribe consciousness to the Earth as it orbits the Sun, or a boulder as it rolls down a hill.

They allegedly escape this apparent intractable difficulty by espousing materialism.  Materialists advocate something of highly questionable intelligibility.  They maintain that consciousness is quite literally the very same thing as certain material processes.  Examples are behaviourists who hold that consciousness is literally identical to behaviour.  Or identity theorists who hold that consciousness is literally identical to brain processes.  Or functionalists who hold that consciousness is literally identical to the causal role of such brain processes (there are many flavours of materialism). 

In which case, given the behaviour of a person or the material processes occurring in their brains, their consciousness is logically entailed in much the same way that 2 + 2 = 4 is entailed. That we can look or examine the physical processes occurring in the brain and somehow, derive, have complete certitude, that that person is experiencing consciousness.

Incidentally, this is why materialists hold that p-zombies are conceptually incoherent or metaphysically impossible.  Since consciousness is the very same thing as the relevant material processes, then a being who looks like us, and has a working brain like us, metaphysically necessarily must be conscious, just like the area of a perfect circle must necessarily be πr². 

However, the materialist position here just seems to me to be straightforwardly incorrect.  Regardless of whether we are looking at a person's behaviour or the processes occurring in their brains, we could never be absolutely certain that they are having conscious experiences.  How could we?  How does observing any material process allow me to be acquainted and have full knowledge of another person's consciousness?  It just doesn't.  Yes, the material processes might cause consciousness, might somehow elicit consciousness, but it's literally nonsensical to assert that such material processes are the very same thing as consciousness.  And this shouldn't be of any surprise since material processes are cashed out exclusively by their physical properties -- mass, momentum, charge and so on.  Conscious experiences, on the other hand, apparently wholly lack any such physical properties.  Hence, by definition, consciousness cannot be the same as the correlated physical process and therefore there is no identity, nor any necessarily entailed connection, from one to the other.

The obvious alternative is to deny both causal closure and materialism. Instead, and as commonsense dictates, consciousness in and of itself really does play a causal role in the world.  So, for example, the words and sentences you are reading now are the result of my consciousness.  The material processes alone are insufficient.  It is true that, unlike the materialist, we cannot have absolute certainty that others are conscious. But, via the notion that similar causes have similar effects, we can have a very high confidence.