"An Essay in Aid of a Grammar of Assent" is a philosophical work by John Henry Newman, first published in 1870. In this book, Newman explores the nature of faith and the process of belief formation, offering a sophisticated analysis of how individuals come to assent to propositions, particularly in matters of religion.

The central thesis of the book is Newman's argument that assent, or the act of believing, involves more than just logical reasoning or evidence-based argumentation. Instead, he emphasizes the role of what he terms the "illative sense" - a faculty of the mind that enables individuals to make intuitive leaps in belief, even in the absence of conclusive evidence. Newman contends that this faculty operates on a deeper level than mere intellect, drawing on a combination of reason, intuition, and personal experience.

Newman also discusses the distinction between formal logic and what he calls "real apprehension." While formal logic deals with the consistency and coherence of propositions, real apprehension involves grasping the truth and significance of those propositions in a more holistic sense. Newman argues that this form of apprehension is essential for understanding religious truths, which often transcend the boundaries of empirical evidence and logical demonstration.

Throughout the book, Newman engages with various objections to religious belief, including skepticism, atheism, and religious pluralism. He defends the rationality of faith, asserting that while it may not always be possible to provide irrefutable proof for religious propositions, there are still compelling reasons for assenting to them.

"An Essay in Aid of a Grammar of Assent" is characterized by its nuanced exploration of the psychology of belief and its emphasis on the role of personal conviction in matters of faith. It remains a significant work in the fields of epistemology, philosophy of religion, and theology, influencing subsequent thinkers and provoking ongoing discussion about the nature of religious belief and its justification.