Also, frequently quoted (by me at least): from an interview with Yuri Manin (http://www.ams.org/notices/200910/rtx091001268p.pdf):
> I must explain to you how I imagine mathematics. I am an emotional Platonist (not a rational one: there are no rational arguments in favor of Platonism). Somehow or other, for me mathematical research is a discovery, not an invention. I imagine for myself a great castle, or something like that, and you gradually start seeing its contours through the deep mist, and begin to investigate something.