One True Logic is a major contribution to the philosophy of logic. It is the first monograph explicitly to articulate a version of logical monism, the view that there is a single correct foundational logic. Logical monism is opposed to logical pluralism, according to which there are many correct logics. One True Logic provides a critical overview of the monism vs pluralism debate and argues for the former. It defends a particular monism, based on a highly infinitary logic. Breaking new ground on a number of fronts, it offers novel arguments for the genuine logicality of infinitary logic and unifies disparate discussions in the philosophical and logical literature.