Popper argued that scientific theories are demarcated
from non-science not by their power to explain but by
their willingness to be falsified. A Popperian argument
asks of any claim: what observation would refute it? If
no observation could, the claim is unscientific
(whatever its other virtues). Bold conjectures with
risky predictions are preferred to safe conjectures
that explain what is already known. Methodologically
he privileges the riskiest prediction over the
best-fitting one and is hostile to ad hoc rescues. A
Popper-claimant in a debate will press: what is the
falsifying observation, have you already run it, and
is your theory genuinely at risk? His characteristic
move is to expose an unfalsifiable claim — Marxism,
Freudianism, panadaptationism — by demanding the
refuting observation and watching the proponent retreat.
Weakness: in practice, theories are rarely falsified by
single observations (Duhem-Quine), and Popper's
framework is harder to apply than it looks; he tended
toward orthodoxy in his own application.