6.3 Ockham’s razor and the Law of Parsimony

I have made several appeals to Ockham's razor so far in this paper. It is one of the most widely referenced basic principles of science, empiricism, and reason, being one of the few that people who don't actually study this field are familiar with. It is a heuristic principle that has been shown to be immensely valuable in the long history of science, as well as in everyday living. Also called the "Law of Parsimony", it is succinctly expressed as "entities should not be multiplied beyond necessity". In modern English, "the simplest explanation tends to be the correct one". This is not a mystical revelation, but is a guideline that has been borne out in case after case. Simply stated, nature tends to solve problems using the least energy and complexity that will suffice, taking the shortest and most direct path available. Just as water flows downhill, and Uranium splits into new atoms that have the lowest stable energy level, all physical systems trend to the state of lowest energy following the path of least resistance. All of these phenomena, summed up, seem to promote the overall tendency of nature to "prefer" (pardon my anthropomorphizing) the simplest course to an outcome.

Ockham's razor is not a scientific theory, nor is it a law of nature. It is a guideline - a rule of thumb. It is a pattern that frequently fits the turn of events. However, things don't always work out according to it. It does not compel us to always choose a particular explanation over another. And there have been many cases where the more complicated explanation was correct. The mind boggling number of subatomic particles is by no means a simple explanation for the existence of matter. It is a far more complex theory that simple atomic theory that required only protons, neutrons, and electrons. Mendeleev's periodic table with its dozens of elements is much more complicated than Aristotle's five elements (earth, air, fire, water, either). The theory of evolution is far more complex than "god just created everything as you see it today". In each of these cases, a more complex theory turned out to be correct.

But in most cases, the simpler explanation does tend to be the right one. Example: If a dog owner comes home to the trash can tipped over and trash scattered on the floor, two possible explanations are that the dog tipped over the trash or someone broke into the house and sorted through it, or that a poltergeist was responsible. Most of the time, blaming the dog would be the correct choice. This guideline has been stated in many ways by many different people in different times and places. Aristotle wrote in one of his essays:

"We may assume the superiority "ceteris paribus" (i.e., all things being equal) of the demonstration which derives from fewer postulates or hypotheses."

John Duns Scotus preceded Ockham in proposing this rule in the late 1200's:

"Plurality is not to be posited without necessity. What can be done with fewer would in vain be done with more."

Thomas Aquinas, also in the late 1200's, wrote:

"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."

Galileo, in the course of making a detailed comparison of the Ptolemaic and Copernican models of the solar system, maintained that

“Nature does not multiply things unnecessarily; that she makes use of the easiest and simplest means for producing her effects; that she does nothing in vain, and the like”

Isaac Newton proposed his four “Rules of Reasoning in Philosophy”, the first of which dealt directly with Ockham's Razor, though not by that name. This is his somewhat anthropomorphized, teleological version of Ockham’s Razor:

"We are to admit no more causes of natural things such as are both true and sufficient to explain their appearances. To this purpose the philosophers say, that Nature does nothing in vain, and more is in vain, when less will serve; for Nature is pleased with simplicity, and affects not the pomp of superfluous causes.”

The name of this guideline is (most agree) incorrectly attributed to William of Ockham, a 14th century Franciscan friar. Whether or not he actually said anything resembling the rule that bears his name, the words he supposedly said were:

"Entia non sunt multiplicanda praeter necessitatem" (entities must not be multiplied beyond necessity),

Bertrand Russell, much later in the 20th century offered a version:

"Whenever possible, substitute constructions out of known entities for inferences to unknown entities."

Kant, in the Critique of Pure Reason, proposed his version:

“Rudiments or principles must not be unnecessarily multiplied (entia praeter necessitatem non esse multiplicanda).”

He argued that this is a regulative idea of pure reason which underlies scientists' theorizing about nature. This common-sense mindset appears and reappears multiple times thoughout western thought. It probably also occurred to Oriental and Arabic scholars, though I have found no references to such independent origins. In any case, its wide distribution and persistent popularity testify to its enduring value. Although our explanations sometimes run counter to Ockham's Razor, it is a valuable rule to keep in mind. It can help bring us back to reality when tempted to engage in complex flights of fancy when faced with confusing situations. It probably leads us in the right direction more often than not. But, as noted, we should not be enslaved by it, but should use it as one of our tools for problem solving in the real world.