Part I. Assent and Apprehension
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.
Newman refutes the idea that one needs an infallible judgment in order to believe that the church's judgment is infallible. First, he points out that under this logic, one would need a way to infallibly judge the infallible judgment as infallible, and then a way to infallibly judge that judgment, and this would go on ad infinitum or until one accepts a judgement as infallible without itself having an infallible judge. Secondly, Newman argues that we accept things as true without needing infallibility all the time. If a clock has been broken and therefore displayed an incorrect time, we do not need to question its reading when it has been fixed. Similarly if we see a shadowy figure in the distance and misapprehend what it is, when we get up close to recognize it, we do not need to mistrust our judgement when it was wrong before.
PartII. Assent and Inference
The second part further clarified assent by comparing it with inference. The key difference between assent and inference is that assent is unconditional and inference is conditional, i.e., dependent on other propositions or ideas and unable to stand by itself.
For Newman, inference described a proposition that is intrinsically dependent on other propositions. For instance, the statement, "Therefore, the car is red," is clearly dependent on antecedent propositions for its meaning and those propositions would need to be disclosed before one could meaningfully assent. This is an inferential statement as opposed to "The car in front of the house is red," which is an assertion that can be assented to because it can stand on its own.
There are three types of inferences: formal, informal and natural. Formal inference is logic in the deductive sense. For Newman, logic is indeed extremely useful especially in science and in society. However, its real-world applicability is very limited in that its usefulness is circumscribed by its initial assumptions. For Newman, to make logic work, human thought has to be trimmed to very specific and narrow meanings such that logical statements then lose real world applicability.
Informal inference is akin to calculus. In informal inference one reaches a conclusion by considering the accumulation of converging antecedent probabilities. Natural inference is when the individual, in a simple and whole process, grasps the antecedent conditions and conclusions instantaneously. For instance, if one sees smoke, one may instantly infer the presence of fire. Natural inference, in Newman's view, is related to experience or innate ability.
The second part of the Grammar is where Newman introduces the concept of the Illative Sense, which is for Newman the intellectual counterpart of Aristotle's phronesis. It is the faculty of the human mind that closes the logic-gap in concrete situations and thus allowing for assent. Logic/formal inference utilises dependable processes that lead to a certain and firm conclusion in the fields in which it is applied. However, Newman maintained that in concrete life formal incontrovertible proof in favour of a decision is not possible—the best one can achieve is converging probabilities in favour of a conclusion. For Newman it is impossible to attain the concrete existential equivalent of logical certainty. Thus, to close that gap between converging probabilities and full assent, one needs the aid of the Illative Sense to attain certitude in specific situations.
Newman recognised that there are dangers associated with using the Illative Sense. In using it one may become vulnerable to superstition and eccentricity. But superstition is held in check, Newman suggests, by the moral element in the act of faith, that is, holiness, obedience, and the sense of duty will safeguard faith from becoming mere superstition.
