Sometime when infants are around three years old they begin to recognize that the other human beings in their lives are similar to themselves. Whether it is their parents, care takers, or other kinds that they interact with, typically developing children around three will begin to understand that others have the same feelings and emotions that they have. Infants begin to develop what we call theory of mind. It is a momentous and underrated step in human development.
I seem to be experiencing the world. To me, it feels like there is a conscious individual inside my head who has thoughts, feelings, desires, emotions, joys, pleasures, fears, and experiences of physical phenomena (as an aside, the idea of a single conscious self is up for debate). But I can’t prove that any other person has conscious experiences of the world in the way that I feel as though I do. I can see other people who appear to be virtually exactly like me. I see other people who react in ways I would expect myself to react if I was in a similar happy, scary, or boring experience. I can create a reasonable and testable theory which suggests that other people do indeed have conscious experiences of the world even though I can never fully prove it. This is theory of mind, and this is what three year old toddlers do when they start to realize that other people don’t like being hit, don’t like when someone takes their stuff, or would be happy if someone shared a piece of cake with them.
This is a phenomenal super power and without realizing it, we apply this power in most of the ways we think about the social world. In WEIRD countries it underlies our moral and social contracts. As Steven Pinker writes in The Better Angels of Our Nature, “I experience pleasures and pains, and pursue goals in service of them, so I cannot reasonably deny the right of other sentient agents to do the same.” This humanist philosophy develops from our earliest ages and influences how we understand the world. If we were not able to think about others and infer that they have a mind and have the same experiences as we do, our social interactions would be dramatically different, and peaceable democracies might not be possible.