What do rationalists, atheists, egalitarians and polyamorists have in common? I mean, other than often ending up at the same parties?
These are all identities that come from not believing in something. For polyamorists, it's exclusivity in relationships. For egalitarians, it's differences in value between people. For atheists, it's gods. And for rationalists, it's anything that doesn't change your predictions.
However, the negative is a slippery beast. You could say that a religious person's identity also comes from not believing in something: atheism! So to describe this idea in more precise terms, let's return to the concept of additive and subtractive: you can call something additive if it tries to build from nothing to get to the desired result, and subtractive if it tries to start with some existing thing and cut away at it until the desired result is reached.
To see these different approaches in action with respect to belief, consider a scientific vs religious approach to truth. Science begins with a base of small, empirical truths obtained from observation, and attempts to build from that base to big truths about the universe as a whole. Conversely, religion begins with a big truth about the universe – there's a god and he does everything – and attempts to cut that exhaustive belief down into small everyday truths. If you ask why stars explode, a scientist might say "I don't know", while a religious person would be more likely to say say "I know God did it, but I don't know why".
So to resolve that negative from earlier, "not believing in something" in this case means not accepting some particular subtractive truth you are expected to accept as a given and work backwards from. Instead, you attempt to start from nothing and build additively to that truth. And, in the case of these various non-beliefs, find you can't do it.
What god would develop in a society that never had a god? What hierarchy of human life? What sexual mores? The answers would depend mostly on the popular subtractive truths of the time. Not so with additive truths. In Ricky Gervais's words, if all the books were destroyed, a thousand years from now we'd have different holy books but the same science books.
Perhaps this outlook could be considered a feature of rationality, skepticism or the scientific method, but I think of it as the ur-discipline, the belief behind these various non-belief systems. Don't accept truths that you can't build to additively. Take as little as possible for granted. It is not possible to have a belief system without axioms, but treat all axioms with the utmost suspicion. If they cannot be removed, they should at least make the most modest claims possible.
There is something deeply appealing to me about this way of thinking. It's a kind of intellectual asceticism. A cosmic humbleness. Rather than treating the truth as a big book of mysteries given to us to decipher, we treat it as a structure of our own creation; small at the base, but expanding ever outward into the darkness.