So I am finally done teaching summer school and am ready to settle in to my two weeks of ‘vacation’ before the Fall semester begins. Just as I am about to switch on the PS3 I am struck by the following line of argument…let me know what you think of it…
Those who know me know that I am fond of an argumentative strategy that I call ‘deprioritizing’ when it comes to a priori arguments against (or for) materialism. The idea is taken from the police. When something is deprioritized we still recognize it is a legitimate thing but also recognize that it is not a high priority. So if we are deprioritize the a priori arguments we can still acknowledge that in principle we can tell a priori what is what but for us it will be an empirical discovery. By the time a priori methods will be useful it will be too late. I do this by introducing shombies and zoombies. A shombie is a physical duplicate of me that has consciousness in the absence of any non-material properties. I have claimed that when we are conceiving of a shombie world we are NOT conceiving a a zombie world. But how do we know that it is not? I tend to think of the shombie world as the close possible world where some kind of higher-order theory is true and we have consciousness just like we do in the actual worlds.
This got me to thinking. How does the other side know that consciousness is absent at the zombie world? According to them to know that one is consciously seeing red is to be acquainted with a red quale in such a way as to have it partly constituting my belief or judgment. So to know that we have consciousness, or to know that it isn’t lacking at the actual world, requires being acquainted with it. So how do we know that it is lacking at the zombie world? Sure can conceive of a word with our physics at some future date but all we can ‘see’ is that there are beings there who look like us, talk like us, etc. It would seem that we have no way to tell from the third-person whether these ‘zombies’ really do lack consciousness and since that is the only way for us to know about zombies we are led to a contradiction. In order to conceive of zombies we must know that they lack consciousness, but it is impossible for us to know that they lack consciousness, thus zombies are inconceivable. We can sum this up in the following argument.
1. If zombies are ideally conceivable then we can know that they (the zombies) lack consciousness
2. We cannot know that they lack consciousness
3. Therefore zombies are not ideally conceivable
An opponent might respond that it is just stipulated that there is no consciousness at the zombie world but this is exactly the reason why physicalist claim that the zombie argument is question begging or that it builds into the very concept of consciousness that it is non-physical.
Richard Brown 
Your argument can also be turned around. The whole point of qualia is that they are private. We thus cannot know if someone (else) has qualia. Thus no ideally conceivable person has qualia. Thus all ideally conceivable people are zombies.
In other words, I think that your argument implies both that no ideally conceivable person is a zombie and, by the similar reasoning of the inverse above, that every ideally conceivable person is a zombie. So I think your argument, together with the inverse, implies that people are not ideally conceivable.
That is, we cannot take a definite position on qualia, for or against, within any ideally conceivable framework.
However, doesn’t physicalism hold that reality is, in principle, ideally conceivable within a completed objective description? If so, then I think we have shown that a physicalism cannot apply to people. A reality that takes a definite position on qualia, to either include or exclude them, is not ideally conceivable.
The fact that we experience qualia is self-evident. However the qualia are not ideally conceivable. We know that we are aware, but we have no concept of what this awareness is. Whatever we point to as a concept is just another thing we are aware of, it is not the awareness itself. If we could conceptualize it, then it wouldn’t be private.