This argument is a posteriori; it starts with the world and argues towards an undefined god. It is also known as the teleological argument, from the Greek 'telos', meaning 'end' or 'purpose'.
It comes in three basic forms:
- comparing the world to a machine
- the apparent existence of order in the world
- the impossibility of the world having arisen via chance
It's not as much fun as the ontological argument, or as simply big as the cosmological argument.
The first part, world = like machine, was done by William Paley, among others, as mentioned above. He claimed that a person who saw a watch would know it had been designed, as all the parts fitted together to form a working whole. This would apply even to someone who had never seen a watch, or if the watch was malfunctioning. Likewise, the world is made up of functioning parts which form a working whole, and so has been designed (and made. This applies even though we have nothing with which to compare the world, and even though the world sometimes appears to be malfunctioning. Many other scholars advanced this idea, such as David Hume (although he actually disagreed with it) and A. E. Taylor.
The second part is somewhat similar, and can be seen as an elaboration on the first. As F. Tennant argued, the presence of things such as food chains, seasons and the laws of physics make the world seem ordered, meaning that a designer is probable. Tennant, however, argued that this apparent order is not proof in itself, and all parts of the design argument must be considered together even to make the existence of god probable.
Lastly, it has been argued that the world is too complicated simply to arise by chance. Pierre Le Comte Du Nouy was a physicist, not a theologian. However, he argued that the simplest molecule, that of protein, is far too complex to have arisen by chance. He did, however, think it was possible if one allowed for an anti-chance factor. Le Comte Du Nouy did not refer to this anti-chance factor as god, but it could be interpreted in that way.
The design argument has criticisms many and fatal, which deserve a node of their own.