 You might have heard of Occam's razor, either in discussion from a movie or book, or perhaps you took a philosophy class where it was part of the coursework. William of Occam was a 14th century English Franciscan friar and writer on topics of logic who was from the small village of Occam and Surrey. It's believed that William joined the orders at a young age and attended the University of Oxford for some years, but never completed his degree. He was a sort of fundamentalist Franciscan and had rather extreme views on the vow of poverty applying to everyone in the church. Guess who that upset? His views were controversial enough to have charges of heresy laid against him. His conspiracy with other fundamentalist Franciscans led to his excommunication by Pope John 22 and he fled to Bavaria, dying a few years later at a convent in Munich. The heuristic device that bears his name has been a useful tool in the history of science and logic, medicine and chemistry. Why it was named after William is not entirely clear. He was not the first to write on the topic, nor even the most famous. The phrase that he used is less common to us than the modern form. In Latin he stated, plurality ought never be posited without necessity. This has also been restated as entities must not be multiplied beyond necessity, which has sometimes been referred to as the lex parsimmonie, or the law of parsimony, and it is parsimony that Occam's razor is advocating. Let's start with what it is. Parsimony is a stated preference for simplicity in our explanations. When given the choice of hundreds of possible explanations, the law of parsimony suggests that we investigate the simplest explanations first. The fewer forces, or mechanisms, or variables in an equation, or steps in a process we need, the clearer the explanation in general. We refer to it as a razor because, like a very sharp blade, it can be used to remove excess. In this case, remove the unnecessary assumptions from an argument. This is not a new idea. Thomas Aquinas made this argument in the 13th century, writing, If a thing can be done adequately by means of one, it is superfluous to do it by means of several, for we observe that nature does not employ two instruments where one suffices. As the great philosopher of science, Karl Popper once said, we prefer simpler theories to more complex ones, because their empirical content is greater, and because they are better testable. Equally important is what parsimony isn't. It is not a theory, it is not a commandment, it is never a substitute for full explanations that require many forces acting in concert, or an equation requiring many variables. It never takes precedence over the better explanation or good logic. That's important to understand. This is a general guideline for when two explanations are equally explanatory. Many famous people have restated the principle of parsimony over the years. Isaac Newton wrote, We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances. Consider Leonardo da Vinci's elegant statement, Simplicity is the ultimate sophistication. My favorite restatement, though, is from a questionably attributed quote of Einstein. Take everything as simple as possible, but not simpler. There is such a thing as an anti-razor. For example, Walter of Catten, who was a contemporary of William of Occam, devised one that could be stated as, If three things are not enough, to verify an affirmative position about things, a fourth must be added, and so on. How do people use Occam's razor in the real world? Scientists use it as a general heuristic. We look for the simplest explanations first. It's used much more specifically in taxonomy to produce good phylogenetic trees. We try to get the least number of groups balanced against the possibility of overgrouping. The phylogenetic trees are compared and ranked on the basis of their parsimony. This refers to a specific statistical test. Scientists and pathologists have a saying, probably coined by Dr. Theodore Woodward, who won the Nobel Prize for his role in finding cures for typhus and typhoid fever. When you hear hoof beats, think horses, not zebras. In other words, look for the common sources of ailment first before diagnosing rare and exotic conditions. Such exotic causes are referred to in medical circles as zebras. A related dictum is called Sutton's Law, and states then in attempting to diagnose a problem, always do the diagnostic for the most common explanation of illness. The Sutton Law is not named for a famous doctor, but rather Willie Sutton, the bank robber. When asked why he robbed banks, he famously responded, because that's where the money is. Computer science and mathematics deals extensively with parsimony, but since I'm just a biologist, I will leave those topics for someone more qualified. Suppose we take Occam's razor and apply it to UFO abduction, for example. The UFO skeptics will claim Occam's razor supports the elimination of aliens as a possible explanation. While the UFO proponents will state that applying a biological explanation to common experiences during abduction is a violation of Occam's razor. You can see how both sides have something of a point. We could repeat the same process for many arguments about pseudoscience topics. Intelligent design creationists, for example, could argue that given the extraordinary nature of living things, intelligent design is the simplest explanation, and requiring naturalistic explanations for all the complexity in nature is a violation of parsimony. The counter argument from the scientific perspective is that requiring a supernatural explanation is a violation of parsimony because it invokes an undetectable force to explain nature. I could give any number of examples and every conspiracy theory or pseudoscience would be able to play this game of Occam's razor supports our side, not yours. Today after all is a largely subjective quality, and our preference for simplicity is not necessarily reflected in nature. So is the razor worthless in these situations? Not entirely. What is needed is an objective measurement to stand in for simplicity, which is, as I said, a purely subjective concept. I would propose empirical evidence, another way of saying testability. My own version of the razor would read, if two explanations are equal, the one with empirical evidentiary support is best. For example, there is no empirical evidence of alien abduction. There is no empirical evidence of creationism, as in we have no observations of creation and action in a controlled environment. There is no micro-creation, but there is a micro-evolution that is empirically supported. Any number of conspiracy theories and pseudoscience beliefs fail this basic tenet of empirical evidence, even if the proponents believe themselves to have the simpler explanation they lack a testable mechanism. I want to end with a final point. It's pointless to try to use Occam's razor alone to prove or disprove the existence of a god or gods. It doesn't have that kind of power. But when we apply it to real-world problems like determining phylogenies, or optimizing computer searches, or picking the best way to diagnose a disease, it's amazing what a useful heuristic simplicity can be. Thanks for watching.