Of impulse or reason.

For me, years after having a bit of an Ayn Rand obsession, she's a perfect example of somebody who did say some brilliant, insightful things while being wrong at the very base of her philosophy.

She believed that reason should be the only guide to human action. Hume said that reason is and ought only to be a slave to the passions. It turns out that Hume was right on just about every level on that. We actually have research with people who had brain damage that hampered their ability to act on impulse. They could only act on reason. Their lives were destroyed. Imagine living a life wherein choosing between a Red Bull and a Monster is the same as choosing whether or not to bomb Afghanistan after 9/11. That's basically what these people go through several times per day. We also know that our passions come first and our reason kicks in to justify our impulses.

Rand did get a lot of stuff right. She was right about free markets. A lot of socialists might be surprised to know that she said that capitalism with government help, not socialism, was the worst of economic phenomena. I agree with her there. She said that racism is the most primitive form of collectivism in the 1950s, which was also right.

Reason does need to be a major component in our thinking; but, we're all slaves to our passions whether we like it or not. This is both an is and an ought. Hume and Haidt were and are right about how the human brain works and Rand was wrong. That doesn't make her wrong about everything. Far from it. She's just somebody who requires nuance to navigate properly.