What we mean by “consciousness”

In everyday speech, “conscious” can mean awake, attentive or self-aware. Researchers usually mean something narrower: phenomenal consciousness, the fact that there is “something it is like” to be you, to see red or to feel pain. The phrase comes from the philosopher Thomas Nagel’s 1974 essay What Is It Like to Be a Bat?

Two further distinctions run through the literature. Levels of consciousness describe how conscious you are overall (wide awake, drowsy, dreaming, under anaesthesia). Contents describe what you are conscious of at a given moment. Many experiments target contents, because they can be manipulated precisely, for example by flashing an image too briefly to be seen.

The hard problem in one page

In 1995 the philosopher David Chalmers distinguished the “easy problems” of consciousness, explaining how the brain discriminates stimuli, integrates information and controls behaviour, from the hard problem: why any of that processing is accompanied by subjective experience at all. The easy problems are hard in practice but are, in principle, problems of mechanism. The hard problem asks why mechanism should feel like anything.

Responses fall into broad families. Physicalists expect the gap to close as neuroscience matures. Illusionists, such as Keith Frankish and Daniel Dennett, argue that the sense of an extra “inner glow” is itself a cognitive construction. Others conclude that experience must be built into reality more deeply, which leads to panpsychism or idealism (see below). Read more: the hard problem, explained.

Global Workspace Theory

Established-leaning Proposed by the psychologist Bernard Baars in 1988 and developed into the global neuronal workspace by Stanislas Dehaene and Jean-Pierre Changeux, this theory treats consciousness as a kind of broadcasting. Many specialised processes run unconsciously in parallel; information becomes conscious when it wins access to a shared “workspace” and is made available to the whole system, for report, memory and planning.

Its signature prediction is ignition: a sudden, non-linear, brain-wide surge of activity, involving prefrontal and parietal regions, when a stimulus crosses into awareness. Many experiments report such signatures, though their exact link to experience (rather than to reporting it) is disputed. Global Workspace Theory in detail.

Integrated Information Theory

Debated Developed by the neuroscientist Giulio Tononi from 2004, IIT starts from features of experience itself (it is unified, specific, structured) and asks what a physical system must be like to have them. Its answer is a quantity, Φ (phi), measuring how much a system’s causal structure is integrated beyond its parts. The more integrated information, the more consciousness.

IIT makes bold predictions: experience should be tied mainly to posterior regions of the cortex, and some simple but highly integrated physical systems could have a little consciousness, while some powerful computers might have almost none. In 2023 an open letter signed by more than a hundred researchers called IIT “pseudoscience”, prompting a sharp debate about how theories of consciousness should be judged. IIT in plain English.

Higher-order and predictive theories

Debated Higher-order theories, associated with David Rosenthal and Hakwan Lau among others, hold that a mental state is conscious when the mind represents itself as being in that state: awareness of a perception, not just the perception. They tend to emphasise the prefrontal cortex.

Predictive processing views, developed by researchers such as Karl Friston, Andy Clark and Anil Seth, describe the brain as a prediction machine that constantly guesses the causes of its sensory input. In Seth’s version, conscious experience, including the experience of being a self, is a “controlled hallucination” anchored in the body’s regulation of itself.

Quantum approaches

Speculative The best-known quantum proposal is Orch OR (orchestrated objective reduction), developed by the mathematical physicist Roger Penrose and the anaesthesiologist Stuart Hameroff in the 1990s. It links conscious moments to quantum processes in microtubules inside neurons and to a proposed physical collapse of the wave function. The main objection is that the warm, wet brain should destroy delicate quantum states far too quickly. Read our guides to Orch OR and quantum theories of consciousness.

Panpsychism and idealism

Speculative If physical description seems to leave experience out, one option is to make experience fundamental. Panpsychism holds that some form of experience is a basic feature of matter; its great challenge is the “combination problem” of how tiny experiences add up to a unified mind. Idealism goes further and treats mind as primary and matter as its appearance. These are philosophical positions rather than experimental theories. Panpsychism explained.

How the theories are being tested

Neuroscience studies consciousness mainly by contrast: comparing trials where the same stimulus is consciously seen or not, comparing wakefulness with sleep or anaesthesia, and studying patients with disorders of consciousness. A newer approach is the adversarial collaboration: rival theorists agree in advance on experiments whose outcomes would count against their own theory. The first large one, the Cogitate consortium, pitted Global Workspace against IIT; its results, published in Nature in 2025, challenged key predictions of both theories. What the adversarial test found.

The theories side by side

How we grade claims: see the Evidence Meter.