Yes, We Can Assign Probabilities to Counterfactuals
Clarifying Causation, Probability, and Counterfactuals in a Quantum Branching Universe
Sara Imari Walker recently tweeted:
"If causation is predicated on counterfactuals, then it cannot be probabilistic: we cannot assign probabilities to things that don’t happen."1
This statement is provocative but hinges critically on a subtle philosophical choice: viewing "things that don’t happen" as inherently non-real.
Within our Quantum Branching Universe (QBU) framework, causation and probability find harmonious coexistence precisely because counterfactuals aren't hypothetical imaginings—rather, they are actualized, concrete branches within an objectively real universal wavefunction. Each branch has a measurable weight or Measure, explicitly justifying the assignment of probabilities even to outcomes we do not observe from our current vantage.
In the QBU, what "doesn't happen" from our perspective does indeed happen elsewhere, ensuring that counterfactuals maintain ontological grounding. Thus, we avoid the philosophical dilemma Walker highlights: we do assign probabilities, robustly and rigorously, to these counterfactuals.
Causation, then, becomes a relationship between branches—actual and counterfactual alike—and probability becomes the objective measure of these relationships across the multiverse. Walker's critique is valuable because it underscores why clarity about one's metaphysical assumptions—specifically, the reality of the multiverse—is foundational to any coherent theory of causation and probability.
In short: yes, probabilities can be meaningfully assigned to counterfactuals—if one accepts a richer metaphysical tapestry where those counterfactuals genuinely exist.