Logic and Conversation 1975
HP Grice's foundational pragmatics paper. He likens natural language to formal logic and proposes that effective communication relies on each party abiding by his namesake cooperative principle, which is built up of four (five?) maxims that can either be followed or flouted. I think this article is important to read to understand authors that do discourse analysis in their papers, such as Mary Bucholtz's 1998 work "Why be normal?".