In crossing a heath, suppose I pitched my foot against a stone, and were asked how the stone came to be there; I might possibly answer, that, for anything I knew to the contrary, it had lain there forever: nor would it perhaps be very easy to show the absurdity of this answer. But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place; I should hardly think of the answer I had before given, that for anything I knew, the watch might have always been there. (…) There must have existed, at some time, and at some place or other, an artificer or artificers, who formed [the watch] for the purpose which we find it actually to answer; who comprehended its construction, and designed its use. (…) Every indication of contrivance, every manifestation of design, which existed in the watch, exists in the works of nature; with the difference, on the side of nature, of being greater or more, and that in a degree which exceeds all computation. — William Paley, Natural Theology (1802)
The most famous example of the argument for God’s existence on the basis of design is Paley’s “watchmaker” illustration. The assumption of such an argument is that there are certain complex systems in the natural world that cannot be explained by natural means and therefore require a super-natural designer.
Critics of this argument have pointed out that just because a complex system may not be explicable on natural grounds at the present moment, that doesn’t mean such an explanation doesn’t exist. Future scientific inquiry may uncover one. In other words, the God of these sorts of design arguments is a “God-of-the-gaps,” a God whose work is limited to the gaps in our current scientific understanding of natural processes. As scientific understanding grows, the “gaps” shrink, and thus God Himself becomes irrelevant.