The big idea: We all feel there's a difference between physics and astrology — one is science, one isn't. But try to say exactly what the difference is, and it slips through your fingers.
Where does the line run?: Both make claims about the world. Both point to evidence. So where does the line between real science and fake science actually run?
This is the demarcation problem: how do you tell science apart from pseudo-science? Karl Popper called this the central question of the philosophy of science.
Hold onto this: The problem isn't that astrology is obviously silly. It's that when you try to write down the rule that rules astrology OUT, the same rule often rules good science out too — or lets astrology back in.
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Start with the answers most people reach for first — and watch each one break.
The tell-tale sign: dodging: Watch what an astrologer does when a prediction fails. 'The stars incline, they don't compel.' 'Your rising sign must have interfered.' There's always a reason it still fits.
Science does the opposite: Real science does the opposite: when the prediction fails, the theory is in trouble. That contrast — facing failure vs dodging it — is the clue the next micro turns into a full theory.
Checkpoint — demarcation: In one line: the demarcation problem is that we can tell science from pseudo-science by feel, but not by any simple rule. The best clue so far: science risks being wrong; pseudo-science protects itself from ever being wrong.
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Behind 'what is science?' sits an even bigger question: what is science even for?
Realism — science aims at truth
- Atoms, genes and germs are really out there
- A theory succeeds because it describes the world correctly
- Science slowly uncovers how reality actually is
Anti-realism — science aims at useful predictions
- We can only test what we can observe
- A theory is good if it works — predicts and lets us build
- Whether unseen atoms are 'really real' isn't science's job
Go further — higher-level insight: Here's the sharpest anti-realist point. Many past theories that 'worked' turned out false — doctors once explained fevers by an unseen fluid, and it made decent predictions.
The pessimistic induction: If successful theories can be wildly wrong about the hidden world, maybe 'it works' is all we can ever really claim — not 'it's true'. Naming this worry (the pessimistic induction) is a top-band move.