In the old idealism it was enough to say that the creator geometrises, that the ratios of numbers supplies the foundational harmonies of the universe, and that space can be calibrated to platonic solids, and so on, and they would have seemed to be saying something like the moderns do with their uncanny aptness of mathematics for the physical world, with their mathematical universe, basic symmetries, simplicity the measure of truth and all that other. But the moderns are selective in the kinds of mathematics that are admissible. A modern idealism, to be as insistent on the primacy of mind might need to assign a cosmic role to the prime numbers, for example, something that we feel is otherwise quite unlikely to occur. How do we know this? How do we immediately recognise an uncanniness about certain parts of mathematics, so that if we encountered a listing of the primes in the signals emitted by a new kind of pulsar we would immediately know that we were in the presence of an alien intelligence? This seems immediately obvious, but the precise distinction, say between matter friendly and mind friendly mathematics, hard to frame. What if the continuum is overturned and physical reality thoroughly quantised, might there then be a role for number theory in new physics? And what about algebraic geometry in some new kind of string theory?