Phi (Integrated Information)

Phi (Φ) is the single number IIT uses to quantify consciousness. It measures integrated information: the amount of cause-effect information a system generates as an irreducible whole, above and beyond what its parts generate independently. Computed by partitioning the system and measuring information loss across the minimum information partition, phi equals zero when a system can be cleanly decomposed into independent components and grows as the system becomes more densely interdependent. A system with high phi has vivid, unified, differentiated experience. A system with zero phi is dark inside, no matter how sophisticated its behavior. Phi is substrate-independent — it depends on causal structure, not material composition.