One needs to do a serious amount of reading and writing, and not just thinking, in order to get anywhere serious in mathematics; contrary to public opinion, mathematical breakthroughs are not powered solely (or even primarily) by "Eureka" moments of genius, but are in fact largely a product of hard work, directed of course by experience and intuition.

Terence Tao: Work hard

To think, you have to write. If you're thinking without writing, you only think you're thinking.

Lesli Lamport: Thinking Above the Code