Philosophy and theology have much in common
Opus Majus, Pars II
Moreover, all speculative philosophy has moral philosophy for its end and aim. And since the end imposes a necessity on those things pertaining to the end, as Aristotle says in the second book of the Physics, therefore speculative science always aspires to its own end, and elevates itself to it, and seeks useful paths to this end, and for this reason speculative philosophy is able to prepare the principles of moral philosophy. Thus therefore are the two parts of wisdom related among the unbelieving philosophers; but with Christian students of philosophy moral science apart from other sciences and perfected is theology, which adds to the greater philosophy of the unbelievers the faith of Christ and truths which are in their nature divine. And this end has its own speculation preceding, just as the moral philosophy of the unbelievers has its own.
There is therefore the same relation between the ends in view as between the speculations; but the end, namely, the Christian law, adds to the law of the philosophers the formulated articles of the faith, by which it completes the law of moral philosophy, so that there may be one complete law. For the law of Christ takes and assumes the laws and morals of philosophy, as we are assured by the sacred writers and in the practice of theology and of the Church. Therefore the speculations of Christians preceding their own law must add to the speculation of the other law those things which are able to teach and prove the law of Christ, in order that one complete speculation may arise, whose beginning must be the speculative philosophy of the unbelieving philosophers; and the complement of this must be added to theology in accordance with the peculiar characteristics of the Christian law. And for this reason the complete philosophy among Christians must have a much more profound knowledge of divine things than it has among the unbelieving philosophers; and for this reason Christians ought to consider philosophy as if it had just been discovered, so that they might make it suitable for its own end; and therefore many things must be added in the philosophy of the Christians, which the unbelieving philosophers could not know. And there are reasons of this kind rising in us from the faith and from the authorities of the law and of the sacred writers, who are acquainted with philosophy, and they can form the common points of complete philosophy and theology. And these are recognized by the fact that they must be common to believers and unbelievers, so that they may be so well known, when they are brought forward and proved, that they cannot be denied by the wise and those instructed in the philosophy of the unbelievers. For the unbelieving philosophers are ignorant of many things at present concerning divine matters, and if these were suitably set before them and proved by the principles of the complete philosophy, that is, by the vivacity of reason, which has its origin in the philosophy of the unbelievers, although completed by the faith of Christ, they would receive it without contradiction and would rejoice in regard to the truth set before them, because they are eager for wisdom and are more studious than Christians. I do not say, however, that any one of the special articles of the Christian faith should be received on trial, but there are many common rational truths, which every wise man would easily accept from another, although he might be ignorant of them himself, as every man studious and desirous of knowledge learns many things from another and receives them by rational arguments, although he was formerly ignorant of them.
Those philosophizing should not be surprised, therefore, if they must needs raise philosophy to the level of divine things and theological truths and the authorities of sacred writers, and employ these freely whenever the occasion arises, and prove them when necessary, and by means of these prove other matters; since without doubt philosophy and theology have much in common. The sacred writers not only speak as theologians, but as philosophers, and frequently introduce philosophical subjects. Therefore Christians desiring to complete philosophy ought in their works not only to collect the statements of the philosophers in regard to divine truths, but should advance far beyond to a point where the power of philosophy as a whole may be complete. And for this reason he who completes philosophy by truths of this kind must not on this account be called a theologian, nor must he transcend the bounds of philosophy; since he can handle freely what is common to philosophy and theology and what must be accepted in common by believers and unbelievers.
There are many such matters besides the statements of unbelieving philosophers, which belonging as it were within the limits of philosophy the man philosophizing in the right way should collect, wherever he finds them, and he should assemble them as though they were his own, whether they occur in the books of the sacred writers, or in the books of the philosophers, or in sacred Scripture, or in the histories, or elsewhere. For there is no author who does not besides his main theme introduce incidentally some matters which belong elsewhere; and for this reason there is a linking together of sciences, because each thing in a manner is dependent on another. But every one who handles a subject in the way he should must assign what belongs to it, both what is necessary and what befits its worth; and therefore wherever he finds these things he knows, how to recognize his own, and therefore he must seize them as his own and arrange them in their proper places. For this reason the philosophizing Christian can unite many authorities and various reasons and very many opinions from other writings besides the books of the unbelieving philosophers, provided they belong to philosophy, or are common to it and theology, and must be received in common by unbelievers and believers. If this be not done, there will be no perfecting, but much loss. And not only must this be done to complete philosophy, but because of Christian conscience which must reduce all truth to divine truth that the former may be subject to and serve the latter; also for this reason that the philosophy of the unbelievers is essentially harmful and has no value considered by itself. For philosophy in itself leads to the blindness of hell, and therefore it must be by itself darkness and mist.
Opus Majus, translated by Robert Belle Burke (Philadelphia: University of Pennsylvania Press, 1928), vol. 1, pp. 72-74.