From Nathaniel Branden's (Blumenthal's) appearance on Booknotes in 1989:
"[Alan Greenspan] was a member of our circle brought in by another member of our circle. He was reading 'Atlas Shrugged' as it was being written. I kind of love the story of Alan because it was -- all in its own terms -- a happy story. Because what made it interesting is that when Ayn first met him in an elevator, or met him through this other person, she was kind of negatively impressed. I liked him initially a good deal more than she did. There were reasons she didn't like some of his ideas. He was a Keynesian economically. He wasn't what he is today politically or economically. And he also at that time was a supporter of a then fashionable philosophy called 'logical positivism,' which taught that you can be certain of nothing. You can know nothing for sure. You can't even be certain that you existed.
So he and I would have rather hilarious conversations in which he would try to convince me that one couldn't know for a certainty that one existed. And my task was to see how irrational this was and to prove philosophically that one could have such knowledge for a certainty. Ayn thought I was quite mad to even waste my time talking to a person who would be saying he doesn't know if he exists or not. I kept saying -- listen, this is a very intelligent man. He is a good guy. You're going to see this story is going to have a very happy ending. And eventually he did change his ideas on these various subjects. He did become a very close friend of Ayn Rand. I think that she really cared for him a lot. And they were devoted until Ayn Rand died in 1982."