**Indexical uncertainty** or **aleatoric uncertainty** is uncertainty about which copy one is (e.g. which branch of the multiverse one occupies), in a reality where the observer state does not provide enough information to uniquely disambiguate instances. Uncertainty is indexical iff it cannot be reduced by gaining more information (epistemic uncertainty) or doing more computation (logical uncertainty). Indexical uncertainty leads to subjective randomness.

The irreducible entropy of a dataset is commensurate with the indexical uncertainty of a hypothetically perfect self-supervised model of that dataset.