Consciousness, in philosophy of mind, refers to the subjective quality of experience—what it is like to be something. It encompasses phenomena such as self-awareness, sensation, thought, and emotion. Philosophical inquiry into consciousness addresses its nature, origins, and relationship to the physical world, often grappling with the "hard problem" of explaining why there is subjective experience at all. This entry surveys the core definitions, historical perspectives, major theories, and key problems that have shaped the philosophical discussion of consciousness.
1 Historical Overview
1.1 Ancient and Medieval Conceptions
In ancient Greek thought, consciousness was not treated as a distinct subject but was often interwoven with discussions of the soul (psyche) and perception. Plato distinguished between the rational part of the soul and the appetitive and spirited parts, while Aristotle linked consciousness to the actuality of the senses. The Stoics and Epicureans each developed materialist accounts of mind, with the Stoics emphasizing a central "commanding faculty" (hegemonikon) responsible for awareness and judgment.
Medieval philosophers, particularly in the Islamic and Christian traditions, continued to debate the nature of the soul. Augustine of Hippo explored introspection and the inner self, arguing that the mind’s self-awareness is immediate and certain. Thomas Aquinas, building on Aristotle, held that the soul is the form of the body and that conscious thought involves abstracting intelligible forms from sensory images.
1.1.1 Aristotelian and Scholastic Views
Aristotle defined the soul as the "first actuality" of a living body, and he considered sensation and intellect to be functions of the soul. For him, self-awareness emerges from the capacity to perceive one’s own perceptions. Scholastic philosophers like Aquinas refined this view, distinguishing between the passive intellect (which receives forms) and the active intellect (which abstracts them). Consciousness was understood primarily in terms of perception and intellectual awareness, not as a separate subjective dimension.
1.2 Early Modern Philosophy
The early modern period saw a dramatic shift in the understanding of consciousness, largely due to René Descartes. Descartes made consciousness the defining property of mind, initiating a new focus on introspection and the privacy of mental states.
1.2.1 Descartes and Substance Dualism
Descartes argued that mind and body are two distinct substances: mind is thinking (res cogitans) and unextended, while body is extended (res extensa) and unthinking. He famously concluded "Cogito, ergo sum" (I think, therefore I am) as the foundation of knowledge. For Descartes, consciousness is the essence of mind; to be a mind is to be conscious. This view—substance dualism—raised the notorious problem of how mental and physical substances can interact, especially within a human being.
1.2.2 Locke and the Stream of Consciousness
John Locke departed from Descartes by tying consciousness to personal identity. He defined a person as "a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places." For Locke, consciousness is not a substance but a set of experiences connected by memory. He anticipated the metaphor of the "stream of consciousness" and argued that identity over time depends on the continuity of consciousness.
1.3 19th and 20th Century Developments
The 19th century brought psychology as an empirical science, while philosophy continued to explore the structure and limits of conscious experience.
1.3.1 William James and the Functional Approach
William James described consciousness as a continuous "stream" rather than a sequence of discrete particles. He emphasized its personal, selective, and functional character: consciousness serves the biological purpose of adapting to the environment. James rejected the idea of a separate soul and argued that consciousness is a function of the brain, instrumental in guiding behavior.
1.3.2 Phenomenology (Husserl, Sartre)
Edmund Husserl founded phenomenology as a method for describing the structures of conscious experience from the first-person perspective. He introduced notions such as intentionality (that consciousness is always about something) and the "epoche" (bracketing of assumptions about the external world). Jean-Paul Sartre extended phenomenology to existential themes, arguing that consciousness is essentially negative—it is not a thing but a "nothingness" that transcends the given world. Sartre’s view emphasized radical freedom and self-creation.
2 Major Theories of Consciousness
2.1 Dualist Theories
Dualist theories hold that consciousness is not entirely physical; either it belongs to a non‑physical substance or it is a non‑physical property of physical systems.
2.1.1 Substance Dualism
Substance dualism posits two fundamentally different kinds of substances: minds (conscious, non‑spatial) and bodies (material, spatial). Descartes is the paradigmatic exponent. The main challenge for substance dualism is explaining how an unextended mind can cause changes in an extended body and vice versa.
2.1.1.1 Interactionist Dualism
Interactionist dualism, defended by Descartes himself, claims that mind and body causally interact—for example, the mind causes voluntary bodily movements, and sensory stimulation causes mental perceptions. The pineal gland was proposed as the point of interaction, but no plausible mechanism has been found, leading to criticism. Alternative dualist views include occasionalism (God mediates interactions) and epiphenomenalism (the mind is a causally inert by‑product of brain activity).
2.1.2 Property Dualism
Property dualism holds that consciousness is a non‑physical property of physical systems (typically brains) that cannot be reduced to physical properties. It accepts that the substance is physical, but mental properties are emergent or fundamental. This view is often motivated by the “explanatory gap” (see §3.2). Prominent property dualists include David Chalmers, who argues that consciousness is a fundamental feature of the universe, analogous to mass or charge.
2.2 Materialist Theories
Materialism (or physicalism) holds that consciousness is entirely physical, either identical to brain processes or functionally realized by them.
2.2.1 Identity Theory
The identity theory asserts that mental states are identical to brain states. For example, the experience of pain just is the firing of C‑fibers. This theory was prominent in the mid‑20th century (J. J. C. Smart, U. T. Place). It was criticized for failing to account for the subjective character of experience—the "what‑it‑is‑likeness" that seems left out by mere neural descriptions.
2.2.2 Functionalism
Functionalism defines mental states by their causal roles—inputs, outputs, and relations to other mental states—rather than by their physical composition. Consciousness is whatever plays the right functional role, regardless of the underlying substance. This allows for multiple realizability: consciousness could exist in brains, silicon chips, or alien biology.
2.2.2.1 Computational Functionalism
A specific version of functionalism, computational functionalism, holds that mental states are computational states of a cognitive system. Consciousness arises from the right kind of information processing. This view is central to classical cognitive science and artificial intelligence research. Critics (e.g., John Searle) argue that syntax alone cannot produce semantics or subjectivity (see §4.3.2).
2.3 Idealist and Panpsychist Theories
These views reject the primacy of the physical, instead treating consciousness as fundamental.
2.3.1 Subjective Idealism
Subjective idealism, associated with George Berkeley, holds that physical objects do not exist independently of perception; to be is to be perceived. Consciousness (or mind) is the only reality, and the physical world is a construct of perceptions. For Berkeley, God’s perpetual perception secures the continuity of objects when no finite mind perceives them.
2.3.2 Panpsychism and Russellian Monism
Panpsychism is the view that consciousness is ubiquitous—every fundamental particle has some form of mentality. This is often motivated by the idea that consciousness cannot emerge from wholly non‑conscious components. Russellian monism, a variant, claims that the intrinsic nature of matter is conscious (or proto‑conscious), while the extrinsic nature is described by physics. This offers a middle way between materialism and dualism.
3 Key Philosophical Problems
3.1 The Hard Problem of Consciousness
David Chalmers coined “the hard problem” to distinguish it from the “easy problems” of explaining cognitive functions (attention, memory, behavior). The hard problem asks: why is there subjective experience at all? Why does a physical process feel like something from the inside? Easy problems are tractable through neuroscience and computation, but the hard problem seems resistant to standard scientific explanation.
3.1.1 Explanation vs. Phenomenology
A core issue is that scientific explanations typically describe mechanisms and functions, but consciousness (phenomenology) seems to escape functional description. For example, one can explain how the brain processes color, but not why that processing is accompanied by a subjective experience of redness. Some philosophers (e.g., Daniel Dennett) deny the hard problem is genuine, arguing that once we explain all the functions, nothing is left.
3.2 Qualia and the Explanatory Gap
Qualia (singular: quale) are the raw feels or qualitative aspects of experience, such as the redness of a rose or the pain of a sting. The “explanatory gap” refers to the inability to fully explain qualia in physical terms. Even if we knew all about brain states, we would not know what it is like to have them.
3.2.1 Inverted Spectrum and Mary’s Room
Two famous thought experiments illustrate these issues. The inverted spectrum imagines two people whose color experiences are reversed (e.g., one sees red where the other sees green) but whose behavior is identical. If this is possible, functionalism seems incomplete. Mary’s Room (Frank Jackson) features a color scientist who knows all the physical facts about color vision but has never seen color. Upon seeing red for the first time, she learns something new—a quale. This suggests that physical knowledge is incomplete regarding consciousness.
3.3 The Unity of Consciousness
Ordinary experience presents a single, unified conscious field, yet the brain is a massively parallel system. How does the brain generate a unified stream of consciousness?
3.3.1 Binding Problem and Split‑Brain Cases
The binding problem asks how separate features (color, shape, motion) are integrated into a single object percept. Neural synchrony and binding through attention are proposed solutions. Split‑brain patients (who have had the corpus callosum severed) show that each hemisphere can be independently conscious, raising questions about the unity of consciousness. Does one person become two? The phenomenon suggests that unity is not a metaphysical necessity but a contingent product of neural integration.
3.4 Consciousness and Intentionality
Intentionality is the “aboutness” of mental states—how they point to or represent objects and states of affairs. The relation between consciousness and intentionality is debated. Some (e.g., John Searle) argue that only conscious states can be genuinely intentional; unconscious mental states are derivative. Others (e.g., Franz Brentano) see intentionality as the mark of the mental, but allow that some intentional states (like beliefs) can occur without consciousness.
4 Special Topics in Philosophy of Mind
4.1 Self‑Consciousness and Self‑Awareness
Self‑consciousness is the awareness of oneself as oneself, distinct from others and the environment. It includes recognition of one’s own mental states as one’s own. Philosophical accounts range from Kant’s transcendental unity of apperception to contemporary theories of metacognition.
4.1.1 Higher‑Order Theories
Higher‑order (HO) theories explain conscious states as states that are themselves the object of higher‑order mental representations. A mental state becomes conscious when it is represented by another mental state (usually of the same subject). This contrasts with first‑order theories, which hold that phenomenal consciousness does not require higher‑order representation.
4.1.1.1 Higher‑Order Thought vs. Higher‑Order Perception
Higher‑order thought (HOT) theory, associated with David Rosenthal, holds that a mental state is conscious if it is accompanied by a suitable higher‑order thought about it. Higher‑order perception (HOP) theory, by contrast, claims that the higher‑order state is a kind of inner sense or perception. Both face challenges: the HOT theory seems to require intellectual concepts for consciousness, while HOP theory struggles with the possibility of misperceiving one’s own mental states.
4.2 Consciousness and Mental Causation
If consciousness is non‑physical (as dualists claim) or epiphenomenal (a mere by‑product), then it seems to have no causal power over behavior—a counterintuitive result. Materialist theories avoid this by making consciousness identical to physical states that are causally efficacious. However, property dualism faces the “causal exclusion” argument: if physical causes are sufficient for behavior, then mental properties (if distinct) are redundant. Defenders of mental causation argue that mental properties can be real and causally relevant without being reducible.
4.3 Animal and Artificial Consciousness
The question of whether non‑human entities are conscious raises ethical and ontological issues.
4.3.1 Criteria for Animal Consciousness
Behavioral and neural criteria are often used: responsiveness to pain, learning, mirror self‑recognition, and complex social interactions. Many mammals and birds show signs of sentience, but the boundaries are fuzzy. The precautionary principle suggests that animals with nervous systems similar to humans deserve moral consideration.
4.3.2 Machine Consciousness and the Chinese Room
Could a computer be conscious? John Searle’s Chinese Room argument challenges the claim that running the right program suffices for consciousness. In the thought experiment, a person inside a room manipulates Chinese symbols according to rules but understands nothing—yet from the outside, the room appears to understand Chinese. Searle concludes that syntax alone (computation) cannot produce semantics or genuine subjectivity. Proponents of strong AI reply that the entire system (room plus person) might be conscious, or that the argument misconceives what computation is.
5 Scientific and Interdisciplinary Approaches
5.1 Neuroscience of Consciousness
Modern neuroscience aims to identify the brain mechanisms underlying conscious experience.
5.1.1 Neural Correlates of Consciousness (NCC)
The NCC are the minimal set of neural events sufficient for a specific conscious percept. Techniques such as fMRI, EEG, and single‑neuron recording have implicated areas like the posterior hot zone (including the precuneus and temporo‑parietal junction) and recurrent processing in sensory cortices. The search for NCCs is ongoing and does not yet explain why those neural events produce subjective experience.
5.1.2 Global Workspace Theory
Proposed by Bernard Baars, Global Workspace Theory (GWT) likens consciousness to a “theater” in which a small amount of information is broadcast widely across the brain. Information that gains access to this global workspace becomes conscious; other information remains unconscious. GWT has been influential in cognitive science and has neurobiological support (e.g., the “ignition” of fronto‑parietal networks during conscious perception).
5.2 Cognitive Science and Consciousness
Cognitive science investigates mental processes such as memory, reasoning, and perception, often using computational models.
5.2.1 Attention and Awareness
Attention is the cognitive process of selectively focusing on certain stimuli while ignoring others. There is a close but not perfect correlation between attention and conscious awareness: some unattended stimuli can reach awareness, and some attended stimuli may not. Studies of inattentional blindness (e.g., the “invisible gorilla”) show that attention is necessary for detailed conscious perception. Distinguishing between top‑down (voluntary) and bottom‑up (stimulus‑driven) attention is crucial.
5.3 Phenomenology and Empirical Methodology
Bridging first‑person subjective reports with third‑person objective measures is a methodological challenge.
5.3.1 Neurophenomenology
Coined by Francisco Varela, neurophenomenology integrates first‑person phenomenological accounts (e.g., from trained participants describing their experience) with simultaneous neuroimaging. The goal is to find systematic correlations between experiential categories and neural dynamics. This approach respects the irreducibility of subjective experience while engaging with empirical science, aiming to close the explanatory gap by refining descriptions on both sides.