An Essay in Aid of a Grammar of Assent (commonly abbreviated Grammar of Assent) is Saint John Henry Newman's seminal book on the philosophy of faith. Completed in 1870, the book took Newman 20 years to write, he confided to friends.

Newman's aim was to show that the scientific standards for evidence and assent are too narrow and inapplicable in concrete life. He argued that logic and its conclusions are not transferable to real life decision making as such. As a result, it is inappropriate to judge the validity of assent in concrete faith by conventional logical standards because paper logic is unequal to the task. "Logic is loose at both ends," he said, meaning that the process of logic initially depends on restrictive assumptions and is thus unable to fit its conclusions neatly into real world situations.

Instead, Newman argued our beliefs come from what he called "the illative sense", that which weighs evidence from all kinds of inputs, logical, instinctive, and authoritative, among others, to reach or fall short of assent. “It is the mind that reasons, and that controls its own reasonings, not any technical apparatus of words and propositions. This power of judging and concluding, when in its perfection, I call the Illative Sense.”

The Grammar was an apologia for faith. Newman was concerned with defending faith as a legitimate product of rational human activity—that assent is not contrary to human nature. He wrote this book against the background of British Empiricism which restricted the strength and legitimacy of assent to the evidence presented for it. John Locke, David Hume and John Stuart Mill, a contemporary of Newman, were the primary Empiricists that Newman was engaged with philosophically.

The Grammar is divided into two sections. The first is entitled "Assent and Apprehension", which deals with believing what one does not understand. The second, entitled "Assent and Inference", addresses the issue of believing what cannot be absolutely proven.

Both parts deal with assent or belief. The first part discussed the relationship between assent and apprehension—what level of intelligent appropriation of a teaching is necessary to believe in that teaching. This section ultimately turns on a distinction between apprehension and understanding. Newman's view was that one can believe as long as one apprehends, even if one does not understand. For example, one may not understand the doctrine of the Trinity, i.e., resolve the individual propositions of the doctrine into one clear whole conception, yet legitimately believe it because apprehension is possible without understanding.

Apprehension, according to Newman, is simply an "intelligent acceptance of the idea or of the fact which a proposition enunciates." So while the regular unlearned Christian, or anyone for that matter, may not be able to conceive that God is one and three, the words of the propositions that define the doctrine are clear and intellectually accessible and assent may legitimately follow.

Newman distinguishes been notional and real assent. Notional assent acknowledges an abstract proposition as true, while real assent occurs when one's deeper faculties of reason and judgment directly perceive the truth. Someone notionally ascending to the creed of Christianity might find its apologetics intellectually convincing, but someone who has really assented will have their instincts, emotions and will aligned with Christian doctrine, and so will worship God, and pray. "In its notional assents, the mind contemplates its own creations instead of things. In real [assent] it is directed towards things, represented by the impressions which they have left on the imagination."