From Descartes' transparent mind and Leibniz's mill to Nagel's bat, Searle's Chinese Room, Dennett's deflation, and Chalmers' hard problem.
What is it for there to be 'something it is like' to be a subject — and can experience be explained in physical terms at all?
Consciousness is at once the most immediate thing we know and the hardest to fit into the scientific picture of the world. Everyone can report on their own experience, yet no physical description of neurons seems to capture what it is like, from the inside, to see red or feel pain. Early modern thinkers treated the mind as transparent to itself; phenomenologists made the careful description of experience a rigorous discipline; behaviourists tried to banish the inner theatre altogether. The late twentieth century sharpened the puzzle into the 'hard problem': even a complete account of brain function might leave untouched the question of why any of it is accompanied by subjective feeling. The thinkers here range from those who think experience is irreducible to those who suspect it is a trick the brain plays on itself.
Rationalism
The mind is a thinking substance whose contents are wholly transparent to itself: to think is, by definition, to be conscious of thinking. Consciousness is the mark of the mental and is immediately and indubitably known, unlike the body, which is known only mediately. This sets the modern agenda by making consciousness both certain and metaphysically distinct from matter.
Meditations on First Philosophy (1641).
Empiricism
Locke defines consciousness as 'the perception of what passes in a man's own mind' and makes it central to both mind and personal identity. All the materials of thought come from experience — sensation and reflection — and reflection is precisely the mind's awareness of its own operations. It is by continuity of consciousness, not of substance, that a person remains the same over time.
An Essay Concerning Human Understanding (1689).
Rationalism
Leibniz distinguishes mere perception from apperception (self-aware consciousness) and posits 'petites perceptions', minute unconscious perceptions below the threshold of awareness. His 'mill' argument holds that if we could walk inside a thinking machine enlarged to the size of a mill, we would find only parts pushing one another and never perception itself — so consciousness cannot be explained mechanically. Perception is thus irreducible to the interaction of physical parts.
Monadology (1714), §17; New Essays on Human Understanding.
Empirical psychology / phenomenology (precursor)
The mark of the mental is intentionality: every mental act is directed at an object — one always sees something, believes something, desires something. This 'aboutness' distinguishes consciousness from mere physical states, which are not of or about anything. Brentano thereby reintroduced the intentional structure of consciousness as the central topic for a scientific psychology of mind.
Psychology from an Empirical Standpoint (1874).
Phenomenology
Consciousness must be studied in the first person by 'bracketing' (epoché) assumptions about the external world and describing experience exactly as it is given. All consciousness is consciousness of something, structured by the correlation between the act (noesis) and its object as meant (noema). Phenomenology thus makes rigorous the description of the structures of lived experience rather than explaining consciousness away.
Ideas Pertaining to a Pure Phenomenology (1913); Logical Investigations.
Pragmatism / psychology
Consciousness is not a chain of discrete ideas but a continuous, personal, ever-changing 'stream of thought' with vague fringes and a selective focus. Later James even doubted that 'consciousness' names an entity at all, proposing that it is a function within the flow of 'pure experience'. He insisted psychology must respect experience as it is actually lived, flowing and selective, rather than atomizing it.
The Principles of Psychology (1890); 'Does Consciousness Exist?' (1904).
Ordinary-language / analytic philosophy
The Cartesian picture of consciousness as an inner private theatre is a 'category mistake', the 'ghost in the machine'. Mental terms refer not to hidden inner episodes but to dispositions to behave in publicly observable ways; to be intelligent or in pain is to be apt to act and react in certain patterns. Consciousness is thus not a private inner realm inspected by an inner eye.
The Concept of Mind (1949).
Phenomenology
Consciousness is not a disembodied spectator but is fundamentally embodied — we are conscious of the world through a lived body that is our anchorage in it. Perception is not the mind processing data but the body's practical, pre-reflective engagement with a meaningful world. This bodily intentionality dissolves the sharp Cartesian split between a thinking subject and an extended object.
Phenomenology of Perception (1945).
Analytic philosophy
An organism is conscious if there is 'something it is like' to be that organism — and we cannot know what it is like to be a bat, however much we learn about its physiology. The subjective character of experience is tied to a point of view that objective, third-person science by its nature leaves out. This shows that a purely physicalist account may be systematically incomplete about the essential feature of consciousness.
'What Is It Like to Be a Bat?' (1974).
Analytic philosophy of mind
The 'Chinese Room' argument aims to show that running the right program is not sufficient for understanding: a person manipulating Chinese symbols by rule, with no grasp of their meaning, models a computer, which likewise has only syntax, not semantics. Consciousness and intentionality are real, biological features caused by and realized in the brain — 'biological naturalism' — not a matter of the right software running on any hardware. Strong AI, for Searle, mistakes simulation for the real thing.
'Minds, Brains, and Programs' (1980); The Rediscovery of the Mind (1992).
Analytic philosophy / cognitive science
There is no inner 'Cartesian Theater' where a self watches experience unfold; consciousness is a 'multiple drafts' process, a bundle of parallel computations with no central finish line. What we call qualia and the unified stream are, Dennett argues, a useful fiction the brain generates, and the 'hard problem' is a confusion to be dissolved rather than a mystery to be solved. Consciousness can, in principle, be fully explained in functional, physical terms.
Consciousness Explained (1991).
Analytic philosophy of mind
Chalmers distinguishes the 'easy problems' (explaining functions like discrimination and report) from the 'hard problem': why is any of this accompanied by subjective experience at all? Since a physically identical 'zombie' seems conceivable, experience does not appear to be logically entailed by the physical facts. He proposes taking consciousness as fundamental, perhaps alongside a form of panpsychism, rather than reducing it to physics.
The Conscious Mind (1996); 'Facing Up to the Problem of Consciousness' (1995).