Integrated Information Theory

Integrated Information Theory (IIT) proposes that consciousness is identical to integrated information, quantified as Φ (phi). Developed primarily by Giulio Tononi and collaborators including Christof Koch, Melanie Boly, and Marcello Massimini, the theory takes a fundamentally different approach from most neuroscientific frameworks by starting from the phenomenology of consciousness itself rather than neural correlates. IIT posits five essential properties of every conscious experience—intrinsicality, information, integration, exclusion, and composition—and derives mathematical requirements that any physical system must satisfy to be conscious.

The theory's central mechanism is the integration of information in a way that cannot be reduced to independent parts. A system is conscious to the degree that it generates integrated information above and beyond what its components generate independently. This is captured mathematically by Φ, which measures the irreducibility of a system's cause-effect structure. Crucially, IIT claims that consciousness is the intrinsic causal power of a physical substrate to make a difference to itself, not merely correlated neural activity or information processing.

IIT makes several distinctive predictions, including that consciousness requires specific physical architecture (not just function), that the cerebellum is likely not conscious despite its computational complexity (due to its modular structure), and that consciousness could exist in non-biological substrates if they achieve sufficient integration. The theory has generated both enthusiasm for its rigorous mathematical framework and controversy for its implications, including a form of panpsychism where any system with non-zero Φ possesses some degree of consciousness.