Can you do a post graduate in Math(or some other hard sciences) if you major in a soft science? It seems possible to go from soft sciences to hard sciences. For example one professor in my philosophy department initially did math as an undergraduate. So I am wondering can you go the opposite way, for example from psychology to maths (maybe because Maths seem to be more fun).

(Sorry if I'm asking a repeat question)
I think what you know and how strong your recommendation letters are the most important factors during the application process. Usually people need to take several foundamental courses to have a solid background before conducting research in math. And you also need to have some undergraduate research experience. So a degree in math is the most efficient way to get this. But it can also be achieved even though you don't have a degree in math. My undergraduate degree is business administration but I have taken around 20 math courses including 10 analysis courses and several algebra and geometry courses during my undergraduate study. Currently I am going to start a research master program in pde and applied analysis and I hope I can do research in math in the future.

So yes, you can. But you need to make extra effort to fill up the gaps.