When theists are asked to justify their belief in God, they often respond that God is required to explain some facet of reality that they consider otherwise unexplainable. God explains where the universe comes from, where life comes from, where morals come from, or why there is something rather than nothing. Since science lacks the answers to these questions1, they claim, we must look to religion. Enter God, stage left.
This line of reasoning is, of course, a logical fallacy known as argumentum ad ignorantiam (argument from ignorance), and sometimes called the "God of the gaps" fallacy. The argument's fault lies in the implicit assumption that the God explanation gains automatic credence by virtue of humanity's general ignorance on the subject (or, as is often the case, the personal ignorance of the one advancing the argument). The existence of anything can only be established via direct evidence, and the argument from ignorance offers none.
Ignorance, of course, goes a long way toward explaining why religions exist in the first place. The major religions of today were born in an era when life and death phenomena such a weather, disease, and natural disasters were completely unexplained. Thus myths developed to explain what was then a complete mystery, and those myths grew into religions over the course of generations. Viewed from this perspective, "God of the gaps" has a long and inglorious history -- we now understand weather, disease, and natural disasters (to name just a few) without reference to the supernatural. Nevertheless, gaps in our collected knowledge remain, and probably always will; it is a sad commentary on human rationality that many of us tend to cling to any explanation, however inadequate, rather then accept that we simply do not know.
Argumentum ad ignorantiam is closely related to what Richard Dawkins has dubbed the "argument from personal incredulity". This argument, most often deployed by fundamentalists, states that while, for example, the development of life does have a scientific explanation, it is simply too incredible to be believed. This argument can take forms ranging from the uninformed and willfully ignorant ("I just can't see how something as complex as the eye could have just formed itself") to the philosophically ridiculous ("When I consider the miracle of birth, I just can't believe there's no God"). Needless to say, reality is wholly independent of one's capacity to accept it. The argument from personal incredulity has nothing worthwhile to say about the truth, but does say a great deal about the intellectual laziness of the person making it.
Even were arguments from ignorance and personal incredulity logically valid, they would still suffer from another fatal flaw -- the God explanation always regresses to the very problem it purports to solve. If God explains how the universe (or life) came to exist, how did God come to exist? If God is the source of morality, where did God get his morality? If God explains why there is something rather than nothing, why is there God rather than nothing? God is a cheap answer, a pseudo-explanation fit only for those who are unable or unwilling to think critically. A general rule of science (not to mention common sense), known as the principle of parsimony or Occam's razor, states that we should favor economy of explanation. Thus we can dispense with any hypothesis that simply adds complexity and raises new questions, while lacking any real explanatory power in the final analysis. That seems to describe the God hypothesis quite well, as God explains nothing.
1Science does, in fact, have a great deal to say on these topics, as I will explore in future posts. Intellectually honest religious people tend to emphasise that scientific knowledge on such subjects is inadequate or incomplete; dishonest ones tend to ignore, misrepresent, or remain ignorant of the state of scientific knowledge.