[WT Sharpe]

Neither Berkeley nor Tegmark deny the existence of the physical universe. What Berkeley taught was that objects only exist when there is a mind to perceive them. So do the objects that were in a room when we last looked exist even though we are not there in the room to perceive them? Of course, Berkeley would answer. God is there to perceive them.

Tegmark's argument does not depend on any divine beings, and is a bit different. He argues that rather than mathematical objects existing within reality, reality exists within mathematical objects. He espouses what appears to me to be a subtle variant of Neo-Platonism, in that he thinks all fully-defined mathematical objects have a reality and that different universes, not all of which are conducive to life, reside with the various mathematical structures. At least, that's how I read him.