Psychological continuity and my first psychological state

Assume that human psychological states depend on brain states.

On psychological continuity theories of personal identity, if a time t is such that I did not have a psychological state at t, then I did not exist at t. Now consider the first time t₁ at which I have a psychological state (for simplicity, I am assuming that the set of times at which I have a psychological state is closed at the bottom; otherwise, the argument gets more complicated). Let ψ₁ be that psychological state. I think—this is perhaps a bit controversial, but very intuitive—that whether I have a psychological state at a given time does not depend on what happens after that time. But the brain states that psychological states depend on are, plausibly, unavoidably diachronic.

Since my having ψ₁ at t₁ cannot depend on how things are after t₁, it follows that it must depend on the brain state at and before t₁.

At first sight, this looks like an obvious contradiction for the psychological continuity theorist. Since I had no psychological states before t₁, I didn’t exist before t₁, and so I couldn’t have had any brain state before t₁.

But there is a way out. While I couldn’t exist before t₁, the brain that is now mine could have. What the psychological continuity theorist can say is that my psychological state at t₁ depends largely on the state that this brain had before I came into existence (together with the instantaneous state the brain has at t₁, when I do exist). This is somewhat counterintuitive.

Another way out, of course, is to drop the physicalism, and allow for neurally-ungrounded psychological states.