1 The Role of Diagrammatic Reasoning in Ethical Deliberation DANIEL G. CAMPOS Abstract In the 1903 lecture “What Makes a Reasoning Sound?” Charles Peirce provides a detailed account of the process of ethical deliberation intended to shape right conduct. In the course of doing so he claims that it involves making a resolution of the nature of a plan that he likens to a diagram. Taking this as a cue, in this paper I develop a detailed account of the role of diagrammatic reasoning in the process of ethical deliberation according to Peirce. I argue that some stages of ethical deliberation are in fact closely analogous to mathematical experimentation and in both reasoning processes the imagination plays a crucial role. Besides further expounding the role of experimental imagination in reasoning in general, this account provides a way to understand the relations between mathematics and ethics and between theory and practice, according to Peirce. Keywords: Charles Peirce, mathematics, ethics, imagination, experimentation, diagrams, deliberation. In the 1903 lecture “What Makes a Reasoning Sound?” Charles Peirce provides a detailed account of the process of ethical deliberation intended to shape right conduct. He does this in the context of arguing against the claim that there is no distinction between moral right and wrong. He considered the argument for this claim to be analogous to the argument for the claim that there is no distinction between good and bad reasoning (EP 2:243-245). 1 Though Peirce’s ultimate concern in the lecture is to show that there is a distinction between good and bad reasoning, his discussion of the distinction between moral right and wrong is important in itself for what it reveals about his views on ethics. For the purposes of this paper, the discussion is important because it provides a careful analysis of the reasoning process of ethical deliberation. In this context Peirce claims that such deliberation involves making a resolution of the nature of a plan that he likens to a diagram (EP 2:246). Taking this as a cue, in this paper I develop a