I read most of Paul Graham's essays and the core theories I associate with him are:

1: As a founder it is more important to be competent and productive than to seem competent and productive.

2: It is an advantage to trust your own competence and ignore common practices.

3: It is better to build something for real users than for imaginary users.

4: A great product is more valuable than great marketing.

They still ring true with me. But maybe I am biased and just remember the thesis that I agree with.