In Hume's day the argument from design was often couched in the form of an analogy. The way the universe is structured and works is like a great machine and just as machines are the product of intelligent engineers it's reasonable to assume that the universe is also the product of an intelligent "engineer."
Hume disagreed. He argued that when we see a machine we conclude that it was designed by an intelligent engineer because we have knowledge of other machines which we know to have been designed by intelligent agents. The universe, however, is unique. There's no other universe to which we can compare it and therefore we can say nothing about how it came to be.
We can conclude that this piece of lead will melt at 621 degrees F because there are other pieces of lead whose properties have been studied and we are justified in assuming that this piece of lead is like those pieces.
But as far as we know this universe is the only universe there is so we have nothing else to compare it to. It's unique and therefore we are not justified in analogizing it to anything else.
Hume's argument, however, is unconvincing. As Oxford philosopher Richard Swinburne writes,
[Hume's] objection has the surprising ...consequence that [scientists] cannot reach justified conclusions about such matters as the size, age, rate of expansion, and density of the universe as a whole (because it is a unique object); and also that [scientists] cannot reach conclusions about the origin and development of the human race (because as far as our knowledge goes, it's the only one of it's kind)....uniqueness is relative to description. Every object is unique under some description.Indeed, each human being is in some ways unique and thus, on Hume's argument, cannot be analogized to anything else, but this is silly. Every object has properties similar to other objects - size, mass, inertia, etc. - and the universe is an object in many ways similar to the objects it contains.
Like other objects it has size, mass, density, age, temperature, and so on and can thus be analogized to other objects.
When we study the properties of the universe and find that they are fine-tuned with such extraordinary precision that had they deviated from their actual value by the most infinitesimal amounts the universe either wouldn't exist or wouldn't be a suitable haven for life anywhere within it, we're overwhelmed with the sense that this state of affairs cannot be just luck.
To believe it is luck is similar to believing that a roulette wheel that lands on the same number a 100 times in a row did so just by chance.
Nothing else in our experience that has the degree of precision (if anything else does) of the dozens of basic parameters and constants of the universe (e.g. the strength of gravity or the force of electromagnetic repulsion) is ever believed to have arisen by chance. It's existence is always imputed to an intelligent mind.
Thus, contra Hume, it's reasonable to assume that the universe is also the product, not of chance, but of an intelligent mind.
