The Grammar of Assent, published in 1870, represents Newman's last major work. As a religious epistemology, it provides systematically thought-through answers to questions that had preoccupied him since his early twenties, as he tells us in his 1864 autobiography, Apologia Pro Vita Sua. When elected as a fellow of Oriel College, Oxford, Newman came under the mentorship of John Keble, one of the most renowned and beloved of nineteenth-century Anglicans. Through Keble, Newman became intrigued with the relation between the mind's assent to propositions and the probability of the evidence on which this assent is based. Bishop Butler, the eighteenth-century commentator on the argument from design for God's existence, had already taught Newman that "probability is the guide of life."
Prima facie, therefore, religious assent and probability seem diametrically opposed. The remarks that follow explain the striking originality with which Newman reconciles this opposition.
For his part, Keble, as Newman explained, met this opposition by affirming that, although probable evidence can introduce a person to a religious assent, it is not the chief basis on which the assent is made. That basis is supplied by the force of the individual's acts of faith and love. These "are directed [by the subject] toward an Object"; that object is the person of Christ, in, by, and through whom the acts of faith and love originate. When the probable evidence is received under the influence of their divine object, then faith and love give the evidence a force "sufficient for internal conviction." Accordingly, the adage that probability is the guide of life becomes transformed when it is incorporated into "an argument from Personality, [which is] a form of the argument from Authority." By this argument, Newman understood Keble to mean that the relation of the Christian to Christ is like that of a friend, moved by love to "anticipate his wishes" and to "understand [and act on] his half-words."
Newman was not fully satisfied with Keble's solution, and his malaise led to the Apologia's deeper penetration, which in turn led to key ideas of the Grammar. In his autobiography, Newman draws a vital distinction, repeated in the Grammar, between "certainty" and "certitude." Certainty is a function of propositions reached by formal reasoning or syllogisms, whereas certitude is a habit of mind, which proceeds according to a person's living discernment assessing concrete situations. As to God's existence, we have properly a certitude. It is grounded, not in the probability of discrete units of evidence, but in the "concurring and converging probabilities" of the "assemblage" of the units of evidence. Thus, a single argument for God's existence, such as from design, may not evince an assent on its own merits, given the weight of its counter-arguments. However, when the array of diverse arguments for God's existence are all seen by the mind to fuse under a single and unified object, then the certitude generated "might equal in measure and strength" the certainty "created by the strictest scientific demonstration."
Differing from Keble, Newman would admit that belief in God based on arguments from reason alone does not require faith or love as infused virtues of grace, unlike belief in God as revealed by Christ. Yet both beliefs yield certitude, because grace baptizes the natural habit, as it were, adapting it to truths known only supernaturally. Before we turn to certitude's deeper cause, let us examine how it differs from certainty in another respect.
When we assent to an a priori proposition, which is true by definition, or to a synthetic a priori proposition, which expresses in a mathematical law an empirical datum of the universe (like Newton's second law of motion), our mind retains neither doubt nor difficulty, once of course we grasp the ground of the proposition's self-evidence. But with certitude, the mind may satisfy all doubt, but still retain difficulties. In other words, when we possess a certitude of God's existence, although the mind yields an unconditional assent that removes doubt, nonetheless, because the assent in grounded in probabilities that cannot be expressed as a synthetic a priori, the mind yearns to answer further questions. For Newman, however, the important point remains that "difficulty and doubt are incommensurate." Thus, he avers, "Ten thousand difficulties do not make one doubt," any more than ten thousand ponies make one horse, he adds elsewhere.
In the Grammar, Newman seeks the conditions for truth in informal reasoning, in which the mind attains certitude in enthymemes, syllogisms lacking middle terms. Such reasoning, which accounts for moving probabilities to unconditioned assents that obviate doubt even within difficulties, is found notably, not only in religion, but also in morality. "The power of judging" and concluding in such reasoning, when in its perfection, is what Newman calls the illative sense. In explaining it, we must take pains to draw attention to Newman's emphasis on the attaining of truth as the mind's whole purpose, end, and goal. If, at the outset, the illative sense is not seen as a function of truth, then serious misunderstandings of the Grammar's project arise, such as that it reduces to emotivism and relativism. To those who argue, for instance, that truth does not exist in morality and religion, because these defy formal reasoning, Newman retorts that such arguments themselves defy the "testimony borne to truth by the mind itself." By this he means that the possession of certitude, caused by the illative sense in these matters, propels people to make often heroic sacrifices in the face of even radically hostile disagreements and threats to life and limb. Newman thus shares an affinity with William James, the nineteenth-century founder of American pragmatism, that discernible action for social good in morality and religion constitutes a sound criterion of the truth of the informal reasoning under-girding the action.
Newman perceived a kinship between the illative sense and Aristotle's virtue of phronesis, prudence or practical wisdom. Precisely as a habit of mind, phronesis enables the proficient application of general rules of ethics to particular cases. For Aristotle, a habit represents a modification of a form, whereas a form represents any intelligible essence. Virtues, then, are habits, because they modify the form or essence of the human person, which is our rational self-consciousness. Virtues are habits, because they provide reason with a rule, measure, or criterion by which we can consciously frame our deeds. Moreover, habits not only give us the rule for acting, but because they also seep into our will, or power of free choice, they enable us to restrain, control, and direct our passions into the righteous practice dictated by the rule.
With remarkable originality, Newman extended phronesis as a habit or virtue from matters of practice, where Aristotle confined it, into the speculative mind's criteria for truth, as such, in informal reasoning. Here the illative sense guides the mind, actively living with its own existential content, in "the means for using correctly principles," facts, doctrines, experiences, and testimonies, and in the means for "discerning promptly what conclusion from them is necessary, suitable, or expedient." Such material, "very often of a personal character," guides informal reasoning "from antecedents to consequents" (i.e., throughout its process). As a result, it should not surprise us, Newman observed, that distinct human beings will vary in the judgments that the illative sense renders in morality and religion. Nonetheless, their variance presents no bar, prima facie, to the judgments' truth. As Aquinas affirms about the adept exercise of prudence: some propositions are known as true only by the wise. It follows that variance in morality and religion is not due to any defect of truth, as such, in these matters, but to degrees of virtuous skill in perceiving truth in the distinctively personal character of the mind's informal material. Analogously, many expert scientists will seek the cause of the same empirical problem, but usually only one or a few find it.
Newman must, however, confront a major problem. Given that persons claim sincere certitudes about conflicting moral and religious propositions, do criteria exist by which skilled phronesis can be discriminated from less skilled, on the assumption that the skilled would point to an objectively true proposition? In the example cited above of the single scientist seeing in data what others do not discern, the test for truth is, finally, the power of the insight to predict similar outcomes in other data. Because no such test is available in religion and morality, Newman approached the problem by further analyzing the informal inference that grounds certitude. In the enthymeme, the mind generates assents by implicit proof "beyond any technical rules" of logic. By implicit proof, he means that, although the illative sense may not adequately express itself in words, it nonetheless grasps when the conditions necessary for an assent are fulfilled.
The resulting certitude does not reduce to mere sentiment because the habit of mind that informally grasps the fulfilled conditions is subordinated, as stated earlier, to truth, the mind's end, goal, and purpose. Newman claimed that this instinctive assent grounds a material certitude. Of itself, it is insufficient for full or complex certitude. This obtains when the mind confirms the material certitude by making explicitly conscious the warrant for the truth of the assent. Readily admitting that false certitudes exist, Newman avers that many of these are due, not to false assents of the illative sense as such, but to false acts of the mind, such as inferences from false premises and false inferences from true premises.
Accordingly, the strength of the Grammar's solution to the major problem lies in its demonstrating the analogous nature of truth. Given that truth is the mind's goal in all its assents, truth's criteria in both formal and informal reasoning must be discerned in light of the variations in human reasoning across different fields of experience. The general unavailability of synthetic a priori propositions in morality and religion does not mean that emotivism and relativism inevitability follow. As Plato observed, when seeking the touchstone for goodness, we should look at the deeds of persons whom our judgment deems good. In light of Newman, Plato's rule means that our illative sense can instinctively know authentic goodness when verifying it in action. Given that the illative sense constitutes a habit of the truth-oriented intellect, its judgment about authentically good persons, although subject to variance among individuals, is not reducible to taste. Because morality and religion admit of arguments and counterarguments, complex certitudes can be established by means of them, even if universal agreement does not result. In short, the central lesson of the Grammar confirms the central lesson of Newman's 1854 Idea of a University: that the purpose of education should be the development of those habits by which the human mind, brought to perfection, can reason correctly. When those habits congenially structure the illative sense, morality and religion, however informal, will more likely escape censure as emotivist and relative by the prevailing "dictatorship" denounced by Benedict XVI.
Another question that long occupied Newman concerns the role of conscience in religion. We could say, for instance, that the whole of the Apologia moves as one harmonious symphony of conscience, in which discrete choices sound variations through his personal history on a persistent theme: finding the truth of Christianity. But in the autobiography's final chapter, Newman made statements that may echo discordantly with the canon of Catholicism on the relation between faith and reason. Whereas he was "far from denying the real force" of traditional arguments for God's existence, nonetheless they neither "warm" nor "enlighten" him, nor make "his moral being rejoice." Much to the contrary: when he peered into the world of experience for confirmation of the divine, he saw "no reflection of its Creator." In fact, in considering "the world in its length and breadth," he perceived "a vision to dizzy and appal": "the defeat of the good, the success of evil," "the pervading idolatries," "the progress of things . . . not toward final causes," and "the tokens so faint and broken, of a superintending design." Accordingly, were it not for the solitary voice "speaking so clearly in [his] conscience," he "should be an atheist, or a pantheist, or a polytheist" when peering to the world. Yet this inner voice, which he equates with the voice of God, is as "certain" to him as his "own existence."
In expressing these sentiments, Newman parts company even with his own theory of the convergence of probabilities for God's existence. Instead he drew inspiration from St. Augustine of the Confessions, for whom God is above all "interior intimo meo," more intimately present to him than he is to himself. Yet both approaches to God, convergence and conscience, are forms of informal reasoning, in which the illative sense plays the decisive role. Whereas in convergence it grounds a certitude based on probabilities observed in external experience, in conscience it grounds, based on subjective experience, a certitude that, nonetheless, when he tries to put" its grounds "into logical shape," he finds "a difficulty in doing so in mood and figure to [his] satisfaction." In conscience, then, as in convergence, the mind assents to God's existence, precisely as it forms the conclusion of an enthymeme, which lacks an explicit middle term linking the conclusion to the evidence. This lack, however, is no obstacle to certitude, because the illative sense supplies the missing link through its habitual skill. It implicitly grasps the fulfilment of the conditions necessary for the unconditional assent that the conscience's voice is God's. True to his word, however, Newman, in both the Grammar and detailed notes published after his death, worked up a complex certitude for the autobiography's material certitude. These sources are well worth study, not only as a compelling argument for God's existence, but also as a self-validation of Newman's own religious epistemology.
 Newman, Apo, 121. [All footnotes correspond with the versions of the works linked in the article.]
 Newman, Apo, 121.
 Newman, Apo, 121.
 Newman, Apo, 121.
 Newman, Apo, 122. See also Newman, Grammar, ch. 9, "The Illative Sense."
 Newman, Apo, 122.
 Newman, Apo, 122.
 See Argument from Conscience to the Existence of God According to J. H. Newman, ed. Adrian J. Boekraad and Henry Tristram (Louvain: Éditions Nauwelaerts, 1961), 106.
 Newman, Apo, 332.
 Newman, Grammar, 340.
 Newman, Grammar, 337.
 Newman, Grammar, 347.
 Newman, Grammar, 347ff.
 Thomas Aquinas, Summa Theologia, I–II, q. 94, a. 2, quoting Boethius.
 Newman, unfinished letter to William Froude, (29 Apr. 1879), in Wilfrid Ward, The Life of John Henry Cardinal Newman, vol. II (London: Longmans, Green, 1912), 589; cited in Zeno, Our Way to Certitude (Leiden: Brill, 1957), 21.
 Newman, Grammar, 349.
 Newman, Grammar, 198ff.
 Joseph Maréchal and Bernard Lonergan, two contemporary interpreters of Aquinas, make compelling arguments that the existence of God from reason constitutes a synthetic a priori.
 Newman, Idea, especially the preface.
 Newman, Apo, 334.
 Newman, Apo, 333.
Newman, Grammar, ch. 5, sec. 1. See note 15 above. See also Stephen Fields, "Image and Truth in Newman's Argument for God," Louvain Studies 24 (1999): 191–210.