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.