This article examines J. S. Mill’s philosophical logic as the basis for his political philosophy. In particular, it explores how he understands the logical mechanism of political discourse in his work A System of Logic and how this understanding supports his defense of freedom of thought and discussion in his On Liberty (chapter 2). While it is well known in the scholarly literature that Mill draws on Socratic dialectic to develop his view of political discourse, this article investigates in detail the logic behind the Socratic dialectic, thereby identifying the historical significance of Mill’s defense of liberty of discussion.