There is no consensus theory of consciousness. But several serious proposals attempt to explain not just what the brain does, but why some of what it does is accompanied by subjective experience. These are the leading contenders.
Proposed by neuroscientist Giulio Tononi in 2004, IIT takes a radical approach: instead of starting from the brain and asking how it produces consciousness, it starts from consciousness itself and asks what physical systems could give rise to it.
IIT begins with five axioms — self-evident properties of experience:
From these axioms, IIT derives postulates about what physical systems must do to be conscious.
IIT's signature claim is that consciousness can be measured. The measure is called Φ (phi) — the amount of integrated information a system generates above and beyond its parts.
IIT makes startling claims:
Critics have pushed back hard. In 2023, a group of scholars characterized IIT as unfalsifiable pseudoscience. A 2025 Nature Neuroscience commentary reiterated the concern. Supporters counter that the theory makes testable predictions about which brain regions are necessary for consciousness. The debate is ongoing.
Proposed by cognitive scientist Bernard Baars in 1988 and expanded by neuroscientist Stanislas Dehaene, GWT uses the metaphor of a theater:
Consciousness, in GWT, is global availability. You become conscious of something when the information about it becomes available to many brain systems simultaneously — for verbal report, memory storage, motor planning, and emotional evaluation.
Dehaene's Global Neuronal Workspace model identifies specific brain mechanisms:
Strengths: GWT is empirically grounded. It makes testable predictions about which brain activity patterns correspond to conscious vs. unconscious processing, and many of these predictions have been confirmed. It explains why attention is closely linked to consciousness and why we can only be conscious of a limited amount of information at once.
Limitations: GWT explains the function of consciousness (global broadcast) but doesn't fully address the hard problem. It tells us when something becomes conscious and what consciousness does, but not why global broadcast is accompanied by subjective experience rather than happening "in the dark."
Higher-Order Theories, championed by philosophers David Rosenthal and Hakwan Lau, propose that a mental state becomes conscious when the brain forms a higher-order representation of that state.
You see red (first-order state). You are aware that you see red (higher-order state). According to HOT, it is the second step — the brain representing its own representations — that makes the experience conscious.
HOT explains why we sometimes process information without being aware of it (blindsight, subliminal perception) — the first-order processing happens, but the higher-order representation doesn't form.
In the mid-1990s, physicist Roger Penrose and anesthesiologist Stuart Hameroff proposed the most ambitious — and most controversial — theory linking quantum physics directly to consciousness.
Penrose had argued in The Emperor's New Mind (1989) that human understanding involves processes that are fundamentally non-computable — they cannot be replicated by any algorithm, no matter how complex. If consciousness is non-computable, it cannot be explained by classical neuroscience (which is ultimately computational). Something else must be involved.
Penrose proposed that this "something else" is quantum gravity — specifically, a process he called objective reduction (OR), in which a quantum superposition collapses not through observation but through reaching a threshold of gravitational self-energy. This collapse, Penrose argued, is where non-computability enters physics — and where consciousness might originate.
Hameroff provided the biological substrate: microtubules — cylindrical protein structures inside neurons that form part of the cellular skeleton. Hameroff proposed that microtubules are not just structural scaffolding but quantum computational devices:
Consciousness, in Orch-OR, is not produced by neurons firing. It is produced by quantum gravity events inside neurons.
Against Orch-OR:
In favor of Orch-OR:
Orch-OR remains a minority position. But it is the only theory that attempts to answer the hard problem by grounding consciousness in a specific physical process — quantum gravitational collapse — rather than in functional organization.
| Theory | Consciousness is... | Key prediction | Hard problem? |
|---|---|---|---|
| IIT | Integrated information (Φ) | Posterior cortex is the seat of experience, not prefrontal | Claims to dissolve it |
| GWT | Global broadcast of information | Prefrontal-parietal ignition marks conscious access | Addresses function, not experience |
| HOT | Self-representation of mental states | Prefrontal damage should eliminate consciousness | Reframes it as a representation problem |
| Orch-OR | Quantum gravity collapse in microtubules | Anesthetics should disrupt microtubule quantum states | Claims to solve it via new physics |
In an unprecedented move, IIT and GWT supporters agreed to a structured adversarial collaboration — pre-registering experiments designed to test competing predictions. The first results, published in 2023, tested whether consciousness correlates more with posterior cortex activity (IIT's prediction) or prefrontal ignition (GWT's prediction).
The results were mixed: they found sustained activity in posterior cortex during conscious perception (supporting IIT), but also found some prefrontal involvement. Neither theory was clearly vindicated or falsified. The collaboration continues, representing a mature and productive approach to one of science's hardest questions.