On “A Defense of Common Sense” (1925), featuring Mark, Wes, Seth, and Dylan. Various philosophers will tell you that the only thing you experience is your own ideas, and hence the world outside of your mind is something wholly unknowable, or if it is knowable, it must be because those supposedly physical objects are actually somehow ideas as well.
Moore defends our pre-philosophical certainty that we do know many things about the world against these skeptics and idealists. While some particular observation of the world may turn out to be wrong (e.g. I see a stick that looks bent but is really straight), there’s no justification for doubting all observations, and in fact our entire language for expressing doubt assumes that there are other people, that we live together in a physical world which has existed for many years before humans existed, that our senses and scientific instruments help us observe things about this world, and that physical objects exist and objectively have many of the properties we tend to ascribe to them.
To argue against “common sense” is to argue that “I know that everyone believes these things, but we are unwarranted in doing so.” The first part of this, “I know that everyone believes,” means that the skeptic already believes in other people that have (allegedly mistaken) beliefs, so this skeptical position is self-refuting.
An episode that came up that is relevant to our conversation here is our interview with John Searle.