On how it feels, and affects one's thought process, to be granted above-top-secret security clearance:
First, you'll be exhilarated by some of this new information, and by having it all — so much! incredible! — suddenly available to you. But second, almost as fast, you will feel like a fool for having studied, written, talked about these subjects, criticized and analyzed decisions made by presidents for years without having known of the existence of all this information, which presidents and others had and you didn't, and which must have influenced their decisions in ways you couldn't even guess. In particular, you'll feel foolish for having literally rubbed shoulders for over a decade with some officials and consultants who did have access to all this information you didn't know about and didn't know they had, and you'll be stunned that they kept that secret from you so well.
You will feel like a fool, and that will last for about two weeks. Then, after you've started reading all this daily intelligence input and become used to using what amounts to whole libraries of hidden information, which is much more closely held than mere top secret data, you will forget there ever was a time when you didn't have it, and you'll be aware only of the fact that you have it now and most others don't….and that all those other people are fools.
It's a complex warning from one complicated historical figure to another, both of whom knew things we can hardly imagine about people and events we assume we know everything about. But, on the surface, Ellsberg's description of how it feels to know a secret seems to explain a lot about the last few years.