The Ultimate Ensemble is a speculative theory of everything (TOE),
suggested by Max Tegmark. Related to the Anthropic principle and
Multiverse theories, the Ultimate Ensemble suggests that not only should
worlds corresponding to different sets of initial conditions or different
physical constants be considered real, but also worlds ruled by altogether
different equations. The only postulate in this theory is that all
structures that exist mathematically exist also physically. The Ultimate
Ensemble is formalized as the "Level IV Multiverse" in Tegmark's
Mathematical universe hypothesis.

In those mathematical structures complex enough to contain self-aware
substructures (SASs), these SASs will subjectively perceive themselves as
existing in a physically "real" world. Tegmark observes that this simple
theory, which has no free parameters at all and may thus be preferred over
all other TOE's by Occam's Razor, is not observationally ruled out. See
also String Theory.

The Ultimate Ensemble can be considered a physico-mathematical expression
of the philosophy known as modal realism.  ¶ 03:54