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In Brief

Probability 1: Why There Must Be Intelligent Life in the Universe
By Amir D. Aczel
Harcourt Brace
230 pp.; $22

Amir Aczel's Probability 1 argues that there must be intelligent life on other planets. Here is his argument: The probability of intelligent life arising on a habitable planet near a sun like ours cannot be all that small since here we are on planet Earth—evolved life forms with intelligence. Recent evidence indicates that planets and suns like ours are abundant throughout the universe. Provided there are sufficiently many opportunities for an event to occur, no matter how small the probability so long as it is positive, we can say with a probability arbitrarily close to 1 that the event will occur; that is, it is nearly certain. The probability of intelligent life arising on a planet like ours is not too small and the number of such planets gives all indications of being huge. QED.

Aczel's argument thus balances the probability of an event against the probabilistic resources for bringing about that event. The probabilistic resources for an event are the number of opportunities for its occurrence. Consider a coin tossed ten times. The probability of tossing ten heads in a row is approximately one in a thousand. Now, if all you have are ten coin tosses, then it's unlikely that you'll witness ten heads in a row. But if you have a million coin tosses, it's virtually certain (i.e., the probability is close to 1) that at some point you'll toss ten heads in a row.

The problem with Aczel's argument is that he never addresses the tension between the probability of an event and its probabilistic resources. He blithely assumes that intelligent life isn't all that improbable and that the probabilistic re sources (i.e., hospitable planets for intelligent life) are sufficiently abundant to render intelligent life a virtual certainty. But "the probability of life occurring on a single planet that is already within its star's habitable zone"—the probability so crucial ...

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