Tegmark has formulated the "Ultimate ensemble theory of everything", whose only postulate is that "all structures that exist mathematically exist also physically". This simple theory, with no free parameters at all, suggests that in those structures complex enough to contain self-aware substructures (SASs), these SASs will subjectively perceive themselves as existing in a physically "real" world. This idea is formalized as the "Mathematical universe hypothesis" in his seminal paper The mathematical universe.

References:

http://en.wikipedia.org/wiki/Max_Tegmark

http://arxiv.org/PS_cache/arxiv/pdf/0704/0704.0646v2.pdf

http://space.mit.edu/home/tegmark/