On taking the common distinction between the legal and the ethical as a point of departure, and in an effort to understand Marshall's approach to self-interest, and thereby to his conception of an ethics of commerce, I read three of his essays in the light of some non-technical writings of Frank Hahn and three other Cambridge intellectuals. My larger project connects self-interest and self-deception to a possible ethics of theorizing in economics, and thereby to the ethics of the relationship between the theorist and the theorized, the analyst and the analyzed.