If the predictor is able to predict that you will two-box, because making such a choice matches your rational profile, then so should the predictor be able to predict that someone would one-box in the case where, according to you, they choose to do so irrationally (and are rewarded for their irrationality). This brings into question your assumption that the subject is unable, through their Phase 2 deliberation, to rationally determine the content of the opaque box, and, by extension, also brings into question your judgement that one-boxing is irrational in such circumstances.
The strategy that you advocate for an agent who begins deliberating in Phase 1, while effective, reminds me of this delightful passage in M.R. Ayers’ book The Refutation of Determinism:
“But it is absurd to represent deliberation as a procedure of trying to influence oneself in a particular direction, since if someone knows in which direction he wants to influence himself, his deliberation is already done. Nor is deliberation a process of trying to bring oneself to perform some act successfully, no matter what. The end of deliberation is not to screw oneself up to the point of acting, but to determine rationally which course of action to follow.” p.152
It is worth noting that there is a tradition in decision theory (so-called causal decision theory, or CDT) that is right to reject what might be called naive evidentialism: the view that the mere fact that an act would be evidence of a good outcome gives one a reason to perform it. Consider a variant of the Newcomb scenario (sometimes called the “smoking lesion” case) where a gene both causes a craving for smoking and independently causes cancer. There, the bare fact that abstaining would be evidence of lacking the gene gives one no genuine reason to abstain, since one’s choice is causally downstream of the gene rather than upstream of it.
CDT correctly insists that what matters is causal structure, not mere evidential relevance. The trouble is that CDT then applies an impoverished conception of causation to Newcomb’s problem that recognizes only efficient causation pushing forward from the present moment and that has no place for rational causation as a distinct causal form. The gene in the medical case operates entirely independently of the agent’s deliberative process, bypassing agency altogether. The Newcombian predictor, by contrast, achieves their reliability precisely because rational causation is efficacious: what they track is the very reasoning process that will issue in the choice.
The disanalogy between Newcomb and the medical cases is therefore itself a causal point, which is why resolving it doesn’t require retreating to evidentialism but rather enriching our ontology of causation.
What Ayers’ comment quoted above also highlights is the peculiar structure of rational causation, where an agent’s reasons for acting are upstream from their decision, but the agent’s deliberative process and their psychological dispositions share a common rational source rather than standing in a simple temporal sequence. When viewed in that way, we can say that the predictor’s decision to fill up (or leave empty) the opaque box has the same rational causal source as the agent’s decision: namely their reasons for acting. So, when an agent decides to one-box during Phase 2, while it is true that at that time the predictor had already determined the content of the opaque box, it is not true that this determination was independent from the agent’s rational deliberative process. The predictor’s action and the agent’s rational decision have the same rational source.
