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If you've ever hung around scientists, you've most likely heard one of them say "the best explanation is the simplest one." But is it? From the behavior of ants to the occurrence of tornadoes, the natural world is often quite complex. Why should we assume the simplest explanation is closest to the truth?

This idea is known as Occam's (or Ockham's) razor. It's also referred to as "rule of economy". And it bears a family relationship to the "principle of least astonishment," which holds that if an explanation is too surprising, it's probably not right. The name comes from William of Ockham, a 14th-century scholastic philosopher. He formulated the principle that "entities (实体) should not be multiplied beyond necessity." The philosophical claim is a form of ontological minimalism: we should not invoke entities unless we have evidence that they exist. In other words: don't make stuff up.

In 1687, Isaac Newton expanded on the notion with his concept of a vera causa - a true cause, stating that we should admit only causes that were both true and sufficient to explain natural phenomena. He added that Nature did nothing in vain and Nature was pleased with simplicity. Although Newton was a great scientist, this claim seems odd. Who is to say what "pleases Nature"? Doesn't this guidance assume we know what we are in fact trying to figure out?

Consider the world of Physics filled with explanations that are surprising, unexpected and hard to get your head around. Newton explained light as being made of particles, whereas other scientists explained it as a wave. Quantum mechanics, however, tells us light is both a wave and a particle. Newton's account was simpler, but modern physics tells us the more complex model is closer to the truth.

When we turn to biology, things get even more complicated. Imagine two smokers, both of whom went through a pack a day for 30 years. One gets cancer; the other doesn't. The simplest explanation? For decades the tobacco industry's answer was that smoking doesn't cause cancer. Simple but false. In fact, disease is complex, and we don't yet understand all the factors involved in cancer.

Occam's razor is not a fact or even a theory. It's a metaphysical (形而上学的) principle: an idea held independently of empirical (实证的) evidence. In human affairs, things are more often than not complex. Human motivations are typically multiple. People can be good and bad at the same time, selfish and selfless, depending on circumstances. The shelves of ethicists are filled with books pondering why good people do bad things, and their answers are rarely short and sweet.

Our explanations should match the world as best as we can make them. Science is about allowing things to unfold naturally, and sometimes this means accepting that the truth is not simple, even if it would make our lives easier if it were.