I think it's less that common sense isn't common and more that it is philosophically impossible to objectively prove the subjective, and likewise impossible to subjectively prove the objective. Objectivity is what subjectivity looks like externally, and subjectivity is what objectivity looks like internally. That’s why brain states and mental states correlate so tightly: they’re the same event seen from two angles, not cause and effect.

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it really depends on the context, but yes

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In my experience, people use the term common sense as a smoke screen in a lot of scenarios.

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