The philosopher G.E. Moore first observed that making a statement and then denying that one knows or believes that statement is unacceptable. For example, "It is raining, but I don’t think that" is defective. Across six experiments (n = 600), this study investigates the nature and extent of this unacceptability as a way to adjudicate between alternative theoretical explanations of this defectiveness. Results confirm that Moorean conjunctions are judged more acceptable than semantic contradictions (e.g., "It is raining, but it isn’t") but less so than felicitous conjunctions (e.g., "It is raining, but it’s okay"), and this is so regardless of whether Moorean conjunctions are produced by a person or by an artificial agent such as ChatGPT. However, if the first conjunct is anchored on a perspective other than the speaker’s, Moorean conjunctions increase in acceptability (e.g., "The train arrives at noon, but I think it won’t"). Several features can drive this perspective-shift, yet the acceptability of Moorean conjunctions did not match that of felicitous sentences in our sample. Our findings support the view that the unacceptability is a byproduct of mindreading, a key part of the social cognition that is crucial to using and understanding language.