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Basic Trope: Someone argues against a well-established and proven scientific theory, and thus either considers it false or considers it equal to any other explanation.
Straight: Bob makes his argument against gravity, either verbally or by physically trying to prove gravity wrong.
Bob does this with other theorums, arguing that the world is flat or the center of the solar system or that magnetism doesn't exist or similar. Note that these extended examples of "only a theorum" are dependent on the setting: It only counts if the story takes place on present-day earth or similar world where these theories are indeed theorums. There is a huge difference between a present-day educated character claiming that the earth is flat or the center of the universe and a stone-age shaman making the same claims.
Bob argues that Peano Arithmetic is only a theory.
Downplayed: Bob is skeptical of the well-established theory, but doesn't see the need to strongly argue the point.
Bob lives in a setting where gravity is indeed optional.
Played For Analysis: Bob actually bothers to make a nuanced argument, hopefully including a distinction between the issue of how / if gravity works and why gravity works (or doesn't work) the way it works (or doesn't work).