A very bad theory about theories

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In some venues, this is a familiar trope: a creationist asserts that evolution is “just a theory,” and a science advocate responds, explaining that, for scientists, a “theory” is a thoroughly tested, well established explanation.

Yet, for over 200 years, scientists have used “theory” for conjectures that are not well established, even ones whose truth is rejected, e.g., Lamarck’s theory.   What is going on?