What's the most important philosophical argument you know?

What are some of the most important philosophical arguments you know. I'm looking for clear, concise answers (even if it's a thread reply) and a link to the most important paper/text that contributed to the argument/idea you're discussing.<br><br>I'm working on a syllabus for an online philosophy class that anyone can attend. I want to have some similarities to how philosophy classes are structured at the university, but am also trying to create a unique experience for students that exposes them to a wide range of theories, concepts, and ideas. We'll be covering ethics, meta-ethics, epistemology & metaphysics. Previously, I thought I would structure the course in a more linear manner, covering the basic theories and concepts related to ethics, meta-ethics, epistemology, and metaphysics. My plan now is to instead identify some of the most important philosophical arguments, identify what subfields these arguments belong to, and create units around these arguments that covers the basic theories and concepts that students will need to be familiar with to properly discuss the argument. <br><br>The goal of the course is to provide an accessible-to-a-general-audience introduction to philosophy, that gives students the tools they need to actually participate in philosophical discussions and debates on their own. The first half of the course will cover philosophical tools. The second half will cover the arguments themselves. I'm currently working on the syllabus for the first half of the course, but want to decide on the arguments we'll be covering in the second half. <br><br>I'd love to hear what you believe are the most important philosophical arguments, as well as what you think would be valuable for students to learn from these arguments.