Back in the 1800s, there was a philosophical argument against any activity by "God", based on the idea that the universe was deterministic and causal, like a big mechanical clock, and so there was no room for God to act. God may have started the clock, but He can't mess with it now, because that would have to violate causality (and break the clock).

I've recently (2007) heard biologists on television arguing along these lines re: evolution. God couldn't have been involved in how evolution turned out, because it's all mechanical and causal. Whatever happened was predetermined by the mechanisms involved.

Given what we know about quantum mechanics, chaotic systems, and comets at this point, this argument seems just absurd from a modern scientific point of view:

  • quantum mechanics: The universe just isn't a clockwork. This has been completely accepted for at least 50 years. Every atom is full of quantum random variables, that unpredictably change value. One example is radioactive decay; predicting when a particular unstable atom is going to decay is believed to be in principle impossible. This gives God lots of room to act without obvious violations of causality. (Note that all sorts of stuff is basically random, including the detailed motion of molecules inside living cells.)
  • chaotic systems: This basically means that in many natural systems, small changes can cascade to have big effects. So tiny random fluctuations can have major impacts later. In terms of evolution, it seems to me that the processes that shape the genome over millenia are a good candidate for being a chaotic system. If true, you couldn't predict what the original organisms on earth would evolve into in 2 billion years.
  • comets: It seems pretty well established now that there were occasional catastrophes that wiped out large fractions of all living species. At least one of these has been definitely linked to an impact crater in Mexico. These big, essentially random events surely affected the course of evolution on earth.
    • None of this means there has to be a God, scientifically, but it makes the clockwork universe argument ridiculous (especially regarding evolution).