phrase requests - Word to describe ideology of unfettered belief in rational self-interest

I'm looking for a word that captures the ideology of believing everything can be traced back to rational self-interest. I know that this is a form of weak altruism, and that many ideas of economic rationality, capitalism, and Enlightenment/neoliberal ideologies tend to be rooted in this, but I can't place the exact word for it.

In fact, the definition of neoliberalism comes close, in how it is described as a faith in the value of the free-market in itself. I am looking for the definition of "faith in the fundamentality of human self-interest in itself." A sort of reductionism where cooperation, community, and all those other good things get traced back to self-interest.

This came about in a discussion about neoliberalism. We began to conflate neoliberalism with this idea, which I think has its own word.