Recently we received an e-mail from a reader asking if ID is a "circular argument." He described ID like this:
Complex stuff doesn't just arise spontaneously, it can only arise through ID. Therefore, if we find evidence of complex stuff, that serves as proof it was ID-ed.
In a very rough and rudimentary kind of way, this is not a totally inaccurate description of the basic case for intelligent design. However, it is not a circular argument.
ID is a historical science based upon the principle of uniformitarianism. The principle of uniformitarianism holds that "the present is the key to the past," where we study present day causes that are at work in the world around us. Once we understand the effects of those causes from present-day observations, we can then study the historical record to see if it too contains the known effects of those causes. When we find those effects, we can infer that the cause was at work.
Here's a brief example.
Geology is a classic case of a historical science. We observe in the present day that rivers remove sediment and cut through rock at a rate, of say, 1 mm per 10 years. If we then observe that a river is in a gorge that is 100 meters deep, we might infer that the river has been cutting that gorge for 100,000 years. (1 year/mm * 1000 mm per meter * 100 m = 100,000 years.) So, by observing present day causes -- that a river cuts through a gorge at 1 mm per year -- we can infer that it cut through the entire gorge, and that it took 100,000 years for that to happen.
Darwin used similar reasoning when he made a case for evolution by natural selection in Origin of Species. Darwin observed present-day populations and observed that they contain variations, and that some variations allow organisms to survive and reproduce better than others. He then theorized that if this process went on deep into the past, it might create lots of variation over long periods of time -- even new species. So he used present-day observations to try to explain past events.
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