What has more consciousness: a puppy or a baby? An iPhone 5 or an octopus? For a long time, the question seemed impossible to address. But recently, Giulio Tononi, a neuroscientist at the University of Wisconsin, argued that consciousness can be measured—captured in a single value that he calls Φ, the Greek letter phi.
The intuition behind Tononi’s idea, known as the Integrated Information Theory, is that we experience consciousness when we integrate different sensory inputs. According to Tononi, when you eat ice cream, you cannot separate the taste of the sugar on your tongue from the sensation of the melting liquid coating the inside your mouth. Phi is a measure of the extent to which a given system—for example, a brain circuit—is capable of fusing these distinctive bits of information. The more distinctive the information, and the more specialized and integrated a system is, the higher its phi. To Tononi, phi directly measures consciousness; the higher your phi, the more conscious you are.
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