How do people here feel about Max Tegmark's Mathematical Universe Hypothesis (http://en.wikipedia.org/wiki/Mathematical_universe_hypothesis). I see it as an extreme form of reification (http://en.wikipedia.org/wiki/Reification_(fallacy)). In this sense, it is taking to the extreme a common fallacy I see in many mathematicians' and physicists' way of thinking about the relationship between mathematical theory and reality. In my opinion, the Mathematical Universe Hypothesis is putting the cart before the horse. To me, mathematical theories offer a descriptive account of physical data - physical data is not somehow generated by or determined by mathematical laws which underly reality. Moreover, I would say that much of what mathematics really is can be analysed in terms of psychology and sociology. There is not some mathematical world of ideas (numbers etc.) - numbers are psychological constructs, and therefore the place they exist is within the human brain. Of course, there is one tricky point which is hard to explain in this way. Why does mathematics do such a good job of describing the physical world?