From City of God, Book V, Chapters 9 and 10
But, let these perplexing debatings and disputations of the philosophers go on as they may, we, in order that we may confess the most high and true God Himself, do confess His will, supreme power, and prescience. Neither let us be afraid lest, after all, we do not do by will that which we do by will, because He, whose foreknowledge is infallible, foreknew that we would do it. It was this which Cicero was afraid of, and therefore opposed foreknowledge. The Stoics also maintained that all things do not come to pass by necessity, although they contended that all things happen according to destiny. What is it, then, that Cicero feared in the prescience of future things? Doubtless it was this—that if all future things have been foreknown, they will happen in the order in which they have been foreknown; and if they come to pass in this order, there is a certain order of things foreknown by God; and if a certain order of things, then a certain order of causes, for nothing can happen which is not preceded by some efficient cause. But if there is a certain order of causes according to which everything happens which does happen, then by fate, says he, all things happen which do happen. But if this be so, then is there nothing in our own power, and there is no such thing as freedom of will; and if we grant that, says he, the whole economy of human life is subverted. In vain are laws enacted. In vain are reproaches, praises, chidings, exhortations had recourse to; and there is no justice whatever in the appointment of rewards for the good, and punishments for the wicked. And that consequences so disgraceful, and absurd, and pernicious to humanity may not follow, Cicero chooses to reject the foreknowledge of future things, and shuts up the religious mind to this alternative, to make choice between two things, either that something is in our own power, or that there is foreknowledge—both of which cannot be true; but if the one is affirmed, the other is thereby denied. He therefore, like a truly great and wise man, and one who consulted very much and very skillfully for the good of humanity, of those two chose the freedom of the will, to confirm which he denied the foreknowledge of future things; and thus, wishing to make men free he makes them sacrilegious. But the religious mind chooses both, confesses both, and maintains both by the faith of piety. But how so? Says Cicero; for the knowledge of future things being granted, there follows a chain of consequences which ends in this, that there can be nothing depending on our own free wills. And further, if there is anything depending on our wills, we must go backwards by the same steps of reasoning till we arrive at the conclusion that there is no foreknowledge of future things. For we go backwards through all the steps in the following order:— If there is free will, all things do not happen according to fate; if all things do not happen according to fate, there is not a certain order of causes; and if there is not a certain order of causes, neither is there a certain order of things foreknown by God—for things cannot come to pass except they are preceded by efficient causes,— but, if there is no fixed and certain order of causes foreknown by God, all things cannot be said to happen according as He foreknew that they would happen. And further, if it is not true that all things happen just as they have been foreknown by Him, there is not, says he, in God any foreknowledge of future events.
Now, against the sacrilegious and impious darings of reason, we assert both that God knows all things before they come to pass, and that we do by our free will whatsoever we know and feel to be done by us only because we will it.
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But it does not follow that, though there is for God a certain order of all causes, there must therefore be nothing depending on the free exercise of our own wills, for our wills themselves are included in that order of causes which is certain to God, and is embraced by His foreknowledge, for human wills are also causes of human actions; and He who foreknew all the causes of things would certainly among those causes not have been ignorant of our wills. For even that very concession which Cicero himself makes is enough to refute him in this argument. For what does it help him to say that nothing takes place without a cause, but that every cause is not fatal, there being a fortuitous cause, a natural cause, and a voluntary cause? It is sufficient that he confesses that whatever happens must be preceded by a cause. For we say that those causes which are called fortuitous are not a mere name for the absence of causes, but are only latent, and we attribute them either to the will of the true God, or to that of spirits of some kind or other. And as to natural causes, we by no means separate them from the will of Him who is the author and framer of all nature. But now as to voluntary causes. They are referable either to God, or to angels, or to men, or to animals of whatever description, if indeed those instinctive movements of animals devoid of reason, by which, in accordance with their own nature, they seek or shun various things, are to be called wills. And when I speak of the wills of angels, I mean either the wills of good angels, whom we call the angels of God, or of the wicked angels, whom we call the angels of the devil, or demons. Also by the wills of men I mean the wills either of the good or of the wicked. And from this we conclude that there are no efficient causes of all things which come to pass unless voluntary causes, that is, such as belong to that nature which is the spirit of life. For the air or wind is called spirit, but, inasmuch as it is a body, it is not the spirit of life. The spirit of life, therefore, which quickens all things, and is the creator of every body, and of every created spirit, is God Himself, the uncreated spirit. In His supreme will resides the power which acts on the wills of all created spirits, helping the good, judging the evil, controlling all, granting power to some, not granting it to others. For, as He is the creator of all natures, so also is He the bestower of all powers, not of all wills; for wicked wills are not from Him, being contrary to nature, which is from Him. As to bodies, they are more subject to wills: some to our wills, by which I mean the wills of all living mortal creatures, but more to the wills of men than of beasts. But all of them are most of all subject to the will of God, to whom all wills also are subject, since they have no power except what He has bestowed upon them. The cause of things, therefore, which makes but is made, is God; but all other causes both make and are made. Such are all created spirits, and especially the rational. Material causes, therefore, which may rather be said to be made than to make, are not to be reckoned among efficient causes, because they can only do what the wills of spirits do by them. How, then, does an order of causes which is certain to the foreknowledge of God necessitate that there should be nothing which is dependent on our wills, when our wills themselves have a very important place in the order of causes? Cicero, then, contends with those who call this order of causes fatal, or rather designate this order itself by the name of fate; to which we have an abhorrence, especially on account of the word, which men have become accustomed to understand as meaning what is not true. But, whereas he denies that the order of all causes is most certain, and perfectly clear to the prescience of God, we detest his opinion more than the Stoics do. For he either denies that God exists,— which, indeed, in an assumed personage, he has labored to do, in his book De Natura Deorum,— or if he confesses that He exists, but denies that He is prescient of future things, what is that but just the fool saying in his heart there is no God? For one who is not prescient of all future things is not God.
Wherefore our wills also have just so much power as God willed and foreknew that they should have; and therefore whatever power they have, they have it within most certain limits; and whatever they are to do, they are most assuredly to do, for He whose foreknowledge is infallible foreknew that they would have the power to do it, and would do it.
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It is not the case, therefore, that because God foreknew what would be in the power of our wills, there is for that reason nothing in the power of our wills. For he who foreknew this did not foreknow nothing. Moreover, if He who foreknew what would be in the power of our wills did not foreknow nothing, but something, assuredly, even though He did foreknow, there is something in the power of our wills. Therefore we are by no means compelled, either, retaining the prescience of God, to take away the freedom of the will, or, retaining the freedom of the will, to deny that He is prescient of future things, which is impious. But we embrace both. We faithfully and sincerely confess both. The former, that we may believe well; the latter, that we may live well. For he lives ill who does not believe well concerning God. Wherefore, be it far from us, in order to maintain our freedom, to deny the prescience of Him by whose help we are or shall be free. Consequently, it is not in vain that laws are enacted, and that reproaches, exhortations, praises, and vituperations are had recourse to; for these also He foreknew, and they are of great avail, even as great as He foreknew that they would be of. Prayers, also, are of avail to procure those things which He foreknew that He would grant to those who offered them; and with justice have rewards been appointed for good deeds, and punishments for sins. For a man does not therefore sin because God foreknew that he would sin. Nay, it cannot be doubted but that it is the man himself who sins when he does sin, because He, whose foreknowledge is infallible, foreknew not that fate, or fortune, or something else would sin, but that the man himself would sin, who, if he wills not, sins not. But if he shall not will to sin, even this did God foreknow.