Another Modal Argument for Substance Dualism

The possibility of some person knowing everything about your brain entails that you aren't identical to your brain (or any part of it). I think it is obvious that it is impossible for God [1], an omniscient being, to know everything about me. God cannot, for example, know what it is like to be me, for He'd have to be me in order to know this, and this is clearly impossible. [2] So there is, necessarily, at least one fact about me to which God has no access whatever. Given that God is omniscient, it'll follow that, necessarily, there is at least one fact about me to which no person has access (other than myself, of course). But, equally obvious, it is possible that God knows everything about my brain. So there is a possible world W in which God knows everything about my brain. Suppose I = my brain (or some part of it) in W. Consider the following truism: 

(E) For all x and y, if x = y, then, for any person P, P knows everything about x just in case P knows everything about y. 

So, by (E), God knows everything about me in W. But, as we have seen, it is impossible that God knows everything about me. So there is no possible world in which God knows everything about me. Thus, it is false that God knows everything about me in W. Since this is a contradiction, we must reject our assumption. Hence, I ≠ my brain in W. All identity statements are necessary, whence it follows that I ≠ my brain in the actual world. If I were a material being it seems that I would be my brain, or some part of it (e.g. I wouldn't be that chair, or my arm). Since I am not my brain (or any part of it) I am not a material being. If I am not a material being then I am an immaterial being. Therefore, I am an immaterial being. Q.E.D. 

I take it to be an obvious fact that I have a material body with which I causally interact. This material body is not me, for I am an immaterial being. Therefore, it is an independent substance to which I am "attached." So it seems that something like substance dualism is true. 

[1] If you don't like God-talk, replace "God" with any person. So, it is possible for some person P to be omniscient with respect to your brain. 

[2] Because this knowledge is impossible to any person not identical to me, God’s lack of this knowledge doesn’t take away from His being omniscience anymore than God’s not being able to create a rock so heavy He cannot lift takes away from His omnipotence.