St Thomas Aquinas (1225 – 1274) argued that the apparent order and complexity in the world is proof of a designer and that this designer is God.
William Paley (1743 – 1805) argued that the complexity of the world suggests there is a purpose to it. This suggests there must be a designer, which he said is God.
Paley used a watch to illustrate his point. If he came across a mechanical watch on the ground, he would assume that its many complex parts fitted together for a purpose and that it had not come into existence by chance. There must be a watchmaker.
Strengths of the argument
The argument only comes up with probabilities, therefore it can continue to develop as new discoveries in science come along.
The argument fits well with the biblical stories of creation, whether these are understood literally or symbolically.
Some developments of the argument, eg the anthropic principle provide ways for ideas about evolution and belief in the existence of God to work together.
Weaknesses of the argument
Complexity does not necessarily mean design.
Even if we accept that the world was designed, it cannot be assumed that its designer is God. And if it were designed by God, then the existence of evil and suffering in the world would suggest that the belief that God is all-good is false.