The human tendency to perceive (and sometimes misperceive) consciousness in each other is often overlooked in philosophy. When you talk to another person, of course you realize intellectually that she’s probably conscious and may have this or that specific thought in her head. You’re performing your own version of the Turing test. But beneath all that noisy cognition, beneath the level of language and philosophical argument, perceptual machinery in your brain is quietly constructing a model of a mind and projecting it onto the other person.

This “social perception” is a little like visual perception. The visual machinery in the brain constructs a model of that blue car. You have cognitive access to the perceptual model and therefore you “know” there’s a blue car in front of you. You can’t choose to not know it, or choose to see it as a green bird instead of a blue car. It isn’t an intellectual exercise. Perception is automatic.

One of the more surreal examples of social perception is ventriloquism, which pits perception against cognition. Everyone in the audience knows cognitively that there’s no mind in the puppet’s wooden head, but we still can’t help falling for the illusion.

Spirituality is another example. God is a social perception. Deities, angels, ghosts, devils, and presences are all consequences of the same machinery in the brain constructing models of conscious minds and attributing them to the spaces around us. People “know” these things not because they logically deduce them, but because machinery in the brain constructs the information at a level deeper than cognition, and in a way that doesn’t easily allow for doubt.

The ultimate example may be our attribution of consciousness to ourselves. The brain constructs a self-model, just as it constructs models of other minds. The self-model is more detailed, more continuous, but essentially the same. We’re just as convinced of the existence of our own inner essence as Terri Schiavo’s parents were of their daughter’s consciousness.

This reflex to attribute consciousness has radical philosophical consequences. Almost all speculation on consciousness focuses on personal consciousness—on what good my own consciousness does me. Some argue its primary function is to help integrate large amounts of information in the brain. Others argue it helps process information at greater depth and makes for more intelligent decisions. A few scholars argue that its purpose is to make life aesthetically worth living. In my own scientific publications I’ve suggested that it helps control attentional focus. All of these approaches take consciousness as a private, internal process. That perspective may be valid as far as it goes, but it’s way too limited. It ignores what may be the most important biological consequence of consciousness: its social impact.