Recently, JP has had a long string of posts which in different ways state the same old argument: We see life on earth. Since life, and the conditions for allowing life, are so improbable, our presence here must have been by design. In JP's case the designer is god. His recent post is more of the same.
The refutations of this argument have been numerous. Basically, the design argument results from a fatal flaw in the understanding of chance and probability. This same error led to similar claims about the "Torah Codes", which have been repudiated by mathematicians. I would like to specifically focus on these fallacies.
1. The design argument ignores the fact that an improbable event will actually probably occur when there are a high number of trials or repetitions. We have to account for the entire field of possibilities. This is similar to the tossing of a coin. The chances of getting 10 heads in a row with tens tosses is slim. However, with a million tosses it becomes much more likely, and if a million different people do a million tosses it becomes even more likely-- and does not require any outside intervention or design. Thus, with life, with every microsecond of every day for hundreds of millions of years becoming a trial or coin toss, multiplied by every gene on the surface of the earth, we get an astronomical number of trials, or oppurtunities for mutations. The odds of favorable mutations and evolutionary changes becomes ever more likely, even highly probable.
2. Another flaw is that of the predetermined outcome. I would like to credit Second Son with this analogy. Basically, rarity is not proof of anything. My chances of winning the jackpot of the state lottery are exceeedingly small. So is everybody else's, before the lottery. However, somebody does win. If I then take the winner, and say: well, his chances were so low, say 1 in 50 million to win, and yet he won anyway, it most have been by design or divine intervention-- that would not be valid logic or appropriate use of probability. Similarly, when creationists absurdly behold the "after the fact" reality of an improbable event (like the earth and life on it), they are conceptually and cynically misusing the concepts of probability.