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Qualia are the felt, subjective qualities of experience — the redness of red, the painfulness of pain, the taste of coffee. Thomas Nagel captured the idea with the phrase 'what it is like': there is something it is like to be a bat using echolocation, but nothing it is like to be a thermostat. Phenomenal consciousness refers to this subjective, experiential dimension of mental life, as opposed to access consciousness (information being available for reasoning and report). The puzzle is that qualia seem to resist functional and physical explanation: you could know everything about the neural correlates of color perception and still not know what red looks like.
Read Nagel's 'What Is It Like to Be a Bat?' (1974) as a primary text. Practice distinguishing phenomenal consciousness from access consciousness using examples: a 'blindsight' patient accesses visual information without phenomenal experience; a dreamer has phenomenal experience without normal access.
When you studied the mind-body problem, you encountered the question of how mental states relate to physical states. Qualia sharpen that question into its hardest form. Start with a simple experience: you see a ripe tomato and it looks red to you. Now imagine a neuroscientist who knows every physical fact about light wavelengths, retinal cells, and visual cortex activity. Does that knowledge tell her what red looks like? Thomas Nagel's phrase "what it is like" captures what seems to be left out: there is a subjective, felt character to seeing red — the redness of red as experienced from the inside — and physical descriptions seem to leave that out entirely.
Qualia are these felt, intrinsic qualities of experience. The painfulness of pain, the taste of coffee, the feeling of anxiety — these are all qualia. They have several features philosophers find philosophically puzzling: they seem private (only you have direct access to your own qualia), ineffable (hard to fully describe to someone who hasn't had them), and intrinsic (not defined by their functional role or causal relationships, but by how they feel). Whether these features are genuine or apparent is contested, but they motivate the idea that phenomenal consciousness is a distinct explanandum.
A useful distinction, introduced by Ned Block, separates phenomenal consciousness from access consciousness. Access consciousness is simply information being available for use in reasoning, verbal report, and guiding behavior — a purely functional notion that physicalists can readily explain. Phenomenal consciousness is the felt dimension. The two can come apart: patients with blindsight can accurately respond to visual stimuli (access) while reporting no visual experience (no phenomenal consciousness). This shows that functional availability and subjective experience are distinct, and that it is phenomenal consciousness in particular that generates the hard explanatory puzzle.
The puzzle qualia create for physicalism is known as the explanatory gap: even a complete physical/functional story of the brain seems to leave unexplained why there is any subjective experience at all. This is not a gap in our current knowledge (which we might close with more neuroscience) but, for many philosophers, a conceptual gap — the very concepts of physical function and phenomenal experience seem to belong to different categories. Whether this gap reflects a genuine metaphysical distinction or an illusion generated by the way we think about minds is the central question you will encounter in the topics that build on this one.
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