This thesis argues that Milton’s two major polemics of 1649, The Tenure of Kings and Magistrates and Eikonoklastes, are preoccupied with articulating proper logic and castigating logic Milton views as inferior. I read these two works alongside Artis Logicae Plenior Institutio, Milton’s seldom read logic textbook published in 1672 but written in the years immediately preceding the regicide. Artis Logicae outlines the procedure for creating and deploying proper logic, but it also describes one type of inferior logic, what Milton calls testimony. Arguments from testimony depend entirely on the ethos of the speaker. For Milton, this argumentative practice represented a departure from critical thought. Further, it was the same type of logic used to support arguments for monarchy. In Milton’s political prose of 1649, testimony lies at the center of critique. The preface to Eikonoklastes frames Milton’s reading of Eikon Basilike as an act of degrading poor logic. Before the work can teach the English nation the proper mode of logic, and thus the proper mode of reading and thought, it must point out the faulty foundation of testimony upon which Eikon Basilike is built. In Eikonoklastes, Milton foregrounds his assault on testimony; in The Tenure of Kings and Magistrates, he buries it beneath the surface of polemic. The first edition of the Tenure challenges Presbyterians’ arguments against the right of the people to depose their rulers, drawing upon a range of proofs in support of its position. But when the polemic failed to convince Presbyterians who sought proof from their divines, Milton reoriented his text in the second edition, recasting his tract as a satire of Presbyterian logic. The second edition of the Tenure offers its Presbyterian audience the testimony it desires while undermining the authority that empowers that testimony.