In every part of the universe we observe means adjusted with the nicest artifice to the ends which they are intended to produce; and in the mechanism of a plant, or animal body, admire how every thing is contrived for advancing the two great purposes of nature, the support of the individual, and the propagation of the species. But in these, and in all such objects, we still distinguish the efficient from the final cause of their several motions and organizations. The digestion of the food, the circulation of the blood, and the secretion of the several juices which are drawn from it, are operations all of them necessary for the great purposes of animal life. Yet we never endeavour to account for them from those purposes as from their efficient causes, nor imagine that the blood circulates, or that the food digests of its own accord, and with a view or intention to the purposes of circulation or digestion. The wheels of the watch are all admirably adjusted to the end for which it was made, the pointing of the hour. All their various motions conspire in the nicest manner to produce this effect. If they were endowed with a desire and intention to produce it, they could not do it better. Yet we never ascribe any such desire or intention to them, but to the watch-maker, and we know that they are put into motion by a spring, which intends the effect it produces as little as they do. But though, in accounting for the operations of bodies, we never fail to distinguish in this manner the efficient from the final cause, in accounting for those of the mind we are very apt to confound these two different things with one another. When by natural principles we are led to advance those ends, which a refined and enlightened reason would recommend to us, we are very apt to impute to that reason, as to their efficient cause, the sentiments and actions by which we advance those ends, and to imagine that to be the wisdom of man, which in reality is the wisdom of God. Upon a superficial view, this cause seems sufficient to produce the effects which are ascribed to it; and the system of human nature seems to be more simple and agreeable when all its different operations are in this manner deduced from a single principle.
Smith is utilizing a proof of intelligent design known as the "watchmaker analogy, " which argues that the existence and harmony of natural laws imply a creator in the same way that the intricate mechanisms of a watch imply a watchmaker. In particular, he emphasizes that this argument for intelligent design has a larger scope than the movements of heavenly bodies (planets, stars, etc.), that it extends to the actions of people: because God is benevolent, Smith argues, God has designed people's sentiments to naturally trend towards moral ends. People are not typically aware of this state of affairs, except upon reflection. Smith's particular mindset explains his overall approach to morality: he mostly posits his theories as straightforward observations of human nature, because human nature, designed by God, is inherently moral. Therefore, while there are morally corrupting influences which must be avoided, Smith does not see morality as something which must be taught.