Kames was a necessitarian or determinist, as we say today. But unlike his protégé Hume, who was satisfied with the freedom of action enjoyed by an agent who is not coerced or constrained by external compulsions, Kames thought that if the general public knew that all our choices are necessitated it would harm public morality.

He concluded that God had given man a natural belief in freedom that was needed as the basis for moral responsibility. Although this is a false belief, it would still suffice to support morality because men were unlikely to give it up.

This radical idea has echoes in modern philosophers. Peter Strawson argued that we would be unwilling to give up belief in moral responsibility even if determinism were shown to be true. John Martin Fischer and Alfred Mele share this view.

And Saul Smilansky goes farther. He thinks that philosophers should hide the "truth" about determinism in order to maintain moral responsibility.

Kames concludes that there is but one possible future, the one foreseen by an omniscient God. To deny this is to deny God's power.

From what hath been proved it appears, that the Divine Being decreed all future events: for he who gave such a nature to his creatures, and placed them in such circumstances, as that a certain train of actions must necessarily follow; did certainly resolve or decree, that events should fall out, and men should act as they do. Prescience indeed is not, properly speaking, any cause of events: for events do not happen because they are foreseen, but because they are to happen, they are capable of being foreseen. Though prescience doth not cause, yet it undoubtedly supposes, the certain futurition (as schoolmen speak) of events. And were there not causes that render the existence of future events certain, it would involve a contradiction to maintain, that future events could be certainly foreseen.

In [Kames'] Sketches of the History of Man, the argument here insisted on is brought within a narrow compass.

With respect to instinctive actions, no man I presume thinks there is any freedom: an infant applies to the nipple, and a bird builds a nest, no less necessarily than a stone falls to the ground. With respect to voluntary actions, the necessity is the same, tho’ less apparent at first view. The external action is determined by the will, the will is determined by desire, and desire by what is agreeable or disagreeable. Here is a chain of causes and effects, not one link of which is arbitrary, or under command of the agent. He cannot act but according to his will: he cannot will but according to his desire: he cannot desire but according to what is agreeable or disagreeable in the objects perceived. Nor do these qualities depend on his inclination or fancy: he has no power to make a beautiful woman appear ugly, nor to make a rotten car case smell sweet.

(Essays on the Principles of Morality and Natural Religion, Essay iii: Liberty and Necessity Online Library of Liberty)

Kames argues that God has arranged our (false) beliefs so that we think that some events are contingent (that chance exists).

The Deity is the primary cause of all things. In his infinite mind he formed the great plan of government, which is carried on by laws fixed and immutable. These laws produce a regular train of causes and effects in the moral as well as material world, bringing about those events which are comprehended in the original plan, and admitting the possibility of none other. This universe is a vast machine, winded up and set a-going: the several springs and wheels operate unerringly one upon another: the hand advanceth and the clock strikes, precisely as the artist had determined. Whoever hath just ideas, will see this to be the real theory of the universe; and that other ways there can be no general order, no whole, no plan, no means nor end in its administration. In this plan, man bears his part, and fulfils certain ends for which he was designed. He must be an actor, and must act with consciousness of spontaneity. He exercises thought and reason, and his nature is improved by the due use of these powers. Consequently, it is necessary, that he should have some notion of things depending upon himself to cause, that he may be led to a proper exercise of that activity for which he was designed. But as a sense of necessity would be a perpetual contradiction to that activity, it was well ordered, that his being a necessary agent should be hid from him. To have had his perceptions and ideas formed upon the plan of universal necessity, to have seen himself a part of that great machine, winded up and set a-going by the author of his nature; would have been inconsistent with the part that is allotted him to act. Then indeed the ignava ratio, the inactive doctrine of the Stoics, would have followed. Conceiving no thing to be contingent, or depending upon himself to cause, there would have been no room for forethought about futurity, nor for any sort of industry and care. He would have had no motives to action, but immediate sensations of pleasure and pain. He must have been formed like the brutes, who have no other principle of action but mere instinct. The few instincts he is at present endued with, would have been insufficient. He must have had an instinct to sow, another to reap; he must have had instincts to pursue every conveniency, and perform every office of life. In short, reason and thought could not have been exercised in the way they are, had not man been furnished with a sense of contingency, and been kept in ignorance of his being a necessary agent. Let the philosopher meditate in his closet upon abstract truth; let him be ever so much convinced of the settled necessary train of causes and effects, which leaves nothing, properly speaking, in his power; yet the moment he comes forth into the world, he acts as a free agent. And, what is wonderful, though in this he acts upon a false supposition, yet he is not thereby misled from the ends of action, but, on the contrary, fulfils them to better advantage.

(Essays on the Principles of Morality and Natural Religion, essay iii. Online Library of Liberty)

Comparing the laws that govern human actions with those that govern the actions of matter, they will be found equally operative, and their effects equally necessary. Where the motives to any action are perfectly full, cogent, and clear, the sense of liberty, as we showed before, entirely vanisheth. In other cases, where the field of choice is wider, and where opposite motives counter balance and work a gains teach other, the mind fluctuates for a while, and feels itself more loose: but at last, must as necessarily be determined to the side of the most powerful motive, as the balance, after several vibrations, to the side of the preponderating weight. The laws of mind, and the laws of matter, are in this respect perfectly similar; though, in making the comparison, we are apt to deceive ourselves. In forming a notion of physical necessity, we seldom think of any force, but what hath visibly a full effect. A man in prison, or tied to a post, must remain there: if dragged along, he cannot resist. Whereas motives, which are very different, do not always produce sensible effects. Yet, when the comparison is accurately instituted, the very same thing holds in the actions of matter. A weak motive makes some impression: but, in opposition to one more powerful, it has no effect to determine the mind. In the precise same manner, a small force will not overcome a great resistance; nor an ounce in one scale, counter balance a pound in the other. Comparing together the actions of mind and of matter, similar causes will in both equally produce similar effects.

But admitting all that hath been contended for, of the necessary influence of motives to bring on the choice or last judgment of the understanding, it is urged by Dr. Clarke, that man is still a free agent, because he hath a power of acting or beginning motion according to his will. In this he placeth human liberty, that motives are not physical efficient causes of motion.

Here ie the reason that libertarians want the agent not to be determined by reasons, motives, etc. They fear is means pre-determined, but it does not

Man is a free agent undoubtedly, because he acts as he wills; but he is equally a necessary agent, as being necessarily influenced by motives to act. The motive, according to his own concession, necessarily determines the will; and the will necessarily produces the action, unless it be obstructed by some foreign force. “But,” says he, “it is only a moral necessity which is produced by motives; and a moral necessity is no necessity at all, being consistent with the highest liberty.” The Doctor’s error lies in opposing moral necessity to liberty. Man is a free agent, because he acts according to his own will. He is at the same time a necessary agent, because his will is necessarily influenced by motives. These are perfectly consistent. The laws of action which respect the human mind, are as fixed as those which respect matter. The idea of necessary, certain, unavoidable, equally agrees to both.

(Essays on the Principles of Morality and Natural Religion, essay iii. Online Library of Liberty)