Reading : Western Philosophy V.6 "The Argument from Design"
In his fifth proof, Aquinas argued that God's existence is shown because things in our world appear to have been designed. This passage has "Cleanthes" quote this argument, and then Hume, in the shape of "Philo", attempting to refute it.
Cleanthes says that when we look at the world, we see that things appear to have been designed with a particular function, or purpose. Since we know that man-made things that are built for a purpose have been designed by man, he says that we should naturally infer that non man-made things in the world have also been designed, and that the designer must be God. This is an argument by analogy: we know that condition "C" causes situation "S" to happen, so when I see situation "S", it is reasonable to infer that "C" must have been its cause.
Philo's response is to say that this is all very well if the situations you're looking at are identical (e.g. a stone falling), but that for different situations, the case is less strong, and it gets weaker the more dissimilar two situations are. E.g. we know that a house is designed, but the universe is quite different from a house, so it's not safe to infer that it also was designed. "Every alteration of circumstances occasions a doubt concerning the event; and it requires new experiments to prove certainly that the new circumstances are of no moment or importance"
Philo also makes the point that the universe is a lot bigger than, and outside the scope of, our own experience. "..can a conclusion, with any propriety, be transferred from parts to the whole?" Also: "Stone, wood, brick, iron, brass, have not, at this time, in this minute globe of earth, an order or arrangement without human art and contrivance; therefore the universe could not originally attain its order and arrangement without something similar to human art! But is part of nature a rule for another part very wide of the former? Is it a rule for the whole? Is a very small part a rule for the universe?"
"When two species of objects have always been observed to be cojoined together, I can infer, by custom, the existence of one wherever I see the existence of the other; and this I call an argument from experience. But how this argument can have place, where the objects, as in the present case, are single, individual, without parallel, or specific resemblance, may be difficult to explain."