Shannon Stapleton / Reuters

In 1990, Terri Schiavo suffered a heart attack and fell into a coma. According to her doctors she was in a vegetative state without any possibility of recovery. Her husband wanted her feeding tube removed, so she could be allowed to die, but her parents believed she was still conscious. Their legal battle wound through the courts for years, becoming a media sensation. At one point, President George W. Bush signed a legislative bill specifically to keep her feeding tube in place. The drama ended only when a federal appeals court finally ordered her feeding tube removed. She died in 2005 after fifteen years in a coma.

The autopsy showed that her brain was severely damaged. Among other notable changes, nearly all her cortical pyramidal neurons were gone. Pyramidal neurons are the brain’s long-distance communicators. Without them there is no possibility of a functioning network, and no possibility of anything we would consider human thought.

To me, the Terri Schiavo case shows the powerful tendency of humans to attribute consciousness to each other.

Philosophers sometimes amuse themselves with an intellectual puzzle: I know I’m conscious, but can I ever really know whether someone or something else is conscious? One popular approach is the Turing test, where you talk to an entity and try to figure out whether it’s a machine or a person. But the Turing test doesn’t apply to a case like Terri Schiavo’s, because she couldn’t talk. The Turing test represents intellectual deduction based on evidence, whereas in Terri’s case something more fundamental and emotional was going on. Her parents experienced a gut wrenching, raw perception—they knew she was conscious. They felt her presence in the room when they sat next to her. In trying to explain their certainty to others they sometimes resorted to rationalizations, for example pointing out the subtle movements of her eyes. The doctors offered alternative explanations: Low-level, brainstem circuits must be triggering the occasional random movements. Schiavo’s parents were unimpressed.

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.

I think of human consciousness like a peacock’s tail. All birds have tails. A tail helps to stabilize the bird during flight or maintain its balance while walking. For peacocks, however, the tail has evolved a spectacular new function: social communication. Likewise, I think many animals use some form of consciousness as a self-model, but in the case of humans it seems to have evolved into something spectacularly public. We navigate the social world by projecting minds onto everything around us. We can’t help it. We live in a spirit world alive with perceived consciousness. It binds people together—parents with children, friends with friends. It’s the basis of our social intelligence and therefore of our success as a species. Terri Schiavo’s parents were demonstrating the deep bonds that make us human.

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