Consciousness is the subjective, first-person aspect of mental life—what it is like to have an experience. Philosophers distinguish phenomenal consciousness (subjective feeling) from access consciousness (information availability) and debate whether consciousness is a unified phenomenon or involves multiple distinct properties.
Consciousness is the phenomenon that makes everything else in philosophy of mind worth caring about. When you see a ripe tomato, your visual system processes wavelength information — but there is also *something it is like* to see red. That felt redness, that inner glow of experience, is what philosophers call phenomenal consciousness. It is the first-person, subjective character of experience, sometimes captured by the phrase "what it is like" (coined by Thomas Nagel). A camera detects the same wavelengths without any felt experience — and that difference is the entire puzzle.
Not all uses of "conscious" track phenomenal experience, though. When psychologists say you are "conscious" of a danger or that information is "consciously accessible," they often mean something different: the information is globally available for reasoning, reporting, and guiding behavior. Ned Block called this access consciousness — the functional availability of mental content. You can be access-conscious of something without having rich phenomenal experience of it, and in some theories phenomenal consciousness might come apart from access entirely. Keeping these two senses distinct prevents a great deal of confusion in philosophy of mind debates.
Why is consciousness so hard to explain? From your introduction to philosophy of mind, you learned that mental states can be characterized by their functional roles — their causal relations to inputs, outputs, and other states. Functionalism handles access consciousness well: we can describe what information is available and how it influences behavior. But phenomenal consciousness resists this treatment. You can specify all the functional properties of a system processing red-wavelength light and still not have explained why there is a felt redness at all. This is what David Chalmers called the hard problem of consciousness — explaining not just what the brain does, but why doing it feels like anything.
The debate over whether consciousness is unified or multiple is equally important. Some philosophers argue phenomenal and access consciousness always go together; others think they can dissociate (blindsight patients, for instance, respond accurately to stimuli they report not seeing). There are also questions about whether consciousness comes in degrees, whether non-human animals are conscious, and whether artificial systems could be. Each of these questions turns on what exactly we mean by "consciousness" — which is why getting the definition right at this stage sets the foundation for everything that follows: accounts of qualia, debates about physicalism, and the question of whether consciousness can be explained at all.
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