When we maintain something has no causal efficacy, what we are saying is that it has no causal impact on its environment whatsoever. If some object -- let's say a rock -- has no causal efficacy whatsoever this means that we wouldn't be able to see it since no light could be reflected off it to enter our eyes. Nor could we touch it since the electrons near the surface of the rock would lack any causal power to repel the electrons near the surface of the tips of our fingers. Our hand would pass straight through it! Indeed it would seem that a causally inefficacious rock cannot be distinguished from a non-existent rock. And the same applies to any physical thing or process. That is to say that in order to exist a physical thing or process must possess causal powers. Even if we reject this it surely is the case that, regardless of whether it makes sense to talk about the existence of a causally inefficacious thing or process, we could never ever know about its existence.
Now the reason why so many people are convinced consciousness has no causal efficacy is because they hold that all change in the world originates from unbroken chains of physical causes and effects. This will not only apply to our conscious decisions to behave in a given manner, but will also apply to the progression of our thoughts too. Let's suppose that in the brain we have a physical causal chain
A → B → C → D → E
But each physical event causes a mental event.
So A → a, B → b, C → c, D → d, E → e where a,b,c,d and e all stand for mental events.
Thus we have an apparent chain of thought a → b → c → d → e, but this is in fact an illusion. Hence the direction that my thoughts take when I think something through and reach certain conclusions, is not guided by my developing understanding as a chain of thought unfolds. Indeed it will have nothing whatsoever to do with my understanding. Rather the direction a chain of thought takes is wholly dictated by impersonal physical laws. The direction of our thoughts is wholly imposed from without, in other words by physical laws, just as the Moons orbit is wholly determined by impersonal physical laws (gravitation in the latter case).
I now maintain that this is simply untenable. First of all I would maintain that we know, indeed with complete certainty, of the existence of our own consciousness. One is in direct contact with one's own consciousness, as it were. At this instance, and in the most immediate sense, I now apprehend I'm having certain experiences. Indeed, to have experiences at all, is by definition to be conscious since to have experiences just is to be conscious. So the first thing to say is that if indeed consciousness is wholly causally inefficacious, then it is the only causally inefficacious existent we could know about!
Moreover, when I entertain the thought of the certainty of my own existence, it must have some temporal duration. I might think to myself yes I know I am conscious. Of course it might not be put explicitly in words, but this thought, this conviction, must be of some temporal duration. But if consciousness is wholly casually inefficacious, then this thought as it reaches fruition is caused by neuronal activity rather than caused by any unfolding understanding on my part. But since it is this understanding, this conscious conviction, that justified my certainty, then in denying the casual efficacy of this understanding or conscious conviction, this then means my certainty cannot be justified. But then we reach an absurdity since one cannot possibly be mistaken in one's realization of one's own consciousness. In other words we cannot escape the conclusion that it is one's unfolding understanding which causes my knowledge of my own consciousness.
And if this thought -- yes I am definitely conscious -- is not caused by the underlying neuronal activity (or at least not wholly so), then we know with absolute certainty that consciousness is causally efficacious. Hence there can be no good reason to doubt that this won't also apply to other instances of reaching an understanding by thinking things through. And it will apply to our decisions and our choices. Since our voluntary behaviour invariably follows on from our decisions to act in a certain way, then it would be reasonable to infer that it applies here too. In other words consciousness must necessarily be causally efficacious. Any scientific evidence which seems to suggest otherwise therefore needs to be reappraised.
Of course this is not to deny that the vast majority of our behaviour is on auto-pilot, as it were. When I walk to a pub, I do it automatically. I do not have to initiate a step on every occasion! But which pub I choose to go to, or indeed whether I decide to go out at all, is likely to be initiated by my consciousness.
Having wrote the above I want to stress that even if one is unconvinced the proof works, the notion that consciousness has no causal efficacy is far from unproblematic. It might be thought for example that if consciousness is literally the very same thing as specific processes within the brain (as in a certain interpretation of reductive materialism), then since the latter are causally efficacious it follows the former are causally efficacious too.
Now I have elsewhere in my essay Science, the Afterlife, and the Intelligentsia explained that reductive materialism is simply not compatible with the existence of consciousness. But even leaving this aside, this strategy cannot wholly escape my foregoing argument.
This is because as materialists (of whatever flavour) they necessarily regard the observable physical processes as comprising a complete explanation of all change in the Universe. This includes all the physical processes within the brain including those correlated with conscious processes such as thinking. And such physical processes are directed only via virtue of physical properties as described by the physical sciences. So this entails we need only pay attention to these physical processes and not to the correlated conscious processes to predict someone's chain of thought.
But if someone's chain of thought is entirely predictable and accountable purely through the physical properties of brains, and the conscious angle of meaning and understanding is not required, then a chain of thought doesn't develop according to any understanding at all -- any such understanding is causally redundant. But then we have no justification whatsoever that any of our thought processes could lead to correct conclusions rather than false conclusions. So even if, contra my Science, the Afterlife, and the Intelligentsia essay, reductive materialism can be squared with the existence of consciousness, we could never have any basis whatsoever for supposing our chains of thoughts when thinking things through will lead to true conclusions rather than false conclusions. I submit that this is staggeringly implausible...
8/4/16 Update. Also see my Materialism/Physicalism is incompatible with our ability to reason.
Science, the Afterlife, and the Intelligentsia