Why can the sentence “Mary has three fingers on her right hand” imply that she has only three fingers without literally stating it?

The sentence "Mary has three fingers on her right hand" does not literally mean she has only three fingers; rather, it carries a conversational implicature that she has exactly three. This distinction falls under the field of pragmatics, which studies how meaning is communicated beyond the literal definitions analyzed in semantics.1

This implication is explained by the cooperative principle developed by H.P. Grice, specifically the first maxim of quantity: "Make your contribution as informative as is required." In a standard conversational context, a listener assumes the speaker is being as informative as necessary. If Mary had four or five fingers, stating she has "three" would be under-informative and misleading. Therefore, the listener infers that the speaker is providing the most accurate and relevant information available, implying that the total count is exactly three.21