Did it ever occur to you that you had within you the power to occupy your mind, and do it more profitably than all those readers? Did it ever occur to you to think?
Hazlitt is talking about thinking with a purpose, to solve a problem, to choose a course to pursue. Real thinking, hard thinking, independent thinking, a systemization of all thought. This sounds a lot like Rand's Hierarchy of Conceptualization and Julian Simon's The Ultimate Resource