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Occam's Razor (also Ockham's Razor or any of several other spellings), is a principle attributed to the 14th century English logician and Franciscan friar, William of Ockham that forms the basis of methodological reductionism, also called the principle of parsimony.
In its simplest form, Occam's Razor states that one should not make more assumptions than needed. When multiple explanations are available for a phenomenon, the simplest version is preferred. A charred tree on the ground could be caused by a landing alien ship or a lightning strike. According to Occam's Razor, the lightning strike is the preferred explanation as it requires the fewest assumptions.
The principle is most often expressed as Entia non sunt multiplicanda praeter necessitatem, or "Entities should not be multiplied beyond necessity", but this sentence was written by later authors and is not found in Occam's surviving writings. William wrote, in Latin, Pluralitas non est ponenda sine neccesitate, which translates literally into English as "Plurality should not be posited without necessity".
Dave Beckett of the University of Kent at Canterbury writes: "The medieval rule of parsimony, or principle of economy, frequently used by Ockham came to be known as Ockham's razor." [1]
The principle of Occam's Razor has inspired numerous expressions including: "parsimony of postulates", the "principle of simplicity", the " KISS principle" (keep it simple, stupid), and in some medical schools "When you hear hoofbeats, think horses, not zebras".
A re-statement of Occam's Razor, in more formal terms, is provided by information theory in the form of minimum message length.
Another variant of this law is Thargola's Sword from Nightfall, (originally a short story by Isaac Asimov and later expanded to a novel in conjunction with Robert SilverbergRobert Silverberg (born January 15, 1935 in Brooklyn, NY) is a prolific author best known for writing science fiction, a multiple winner of both the Hugo and Nebula Awards. Silverberg, a voracious reader from childhood on, began submitting stories to the):
Occam's Razor is now usually stated as follows:
As this is ambiguous, Isaac NewtonKneller's portrait of 1689. Sir Isaac Newton ( December 25, 1642 March 20, 1727 by the Julian calendar then in use; or January 4, 1643 March 31, 1727 by the Gregorian calendar) was an English physicist, mathematician, astronomer, philosopher, and alchemis's version may be better:
Occam's Razor was essentially a conviction that natural science (as seen in the works of Aristotle) and theology must split and go in different directions. The 12th and 13th century saw great efforts to unify theology and reason which cumulated with the works of Thomas Aquinas. Ockham rejected Thomas's "unnecessary" entities such as the active intellect, intelligible species and final causes that Thomas had created to construct a system of thought linking the operations of the natural world with the creativity of God. Occham said such a construct was impossible and that the study of the natural world, and the study of theology must split. The former is knowable; the latter will forever be a mystery. The divorce of reason and faith is a major turn in Western thought and significant in the development of scientific thinking.
Occam's Razor is known by several different names including the Principle of Parsimony, The Principle of Simplicity, and The Principle of Economy. The reason for these alternate names can be explained by the association of simplicity and parsimony with Occam's Razor. Prior to the 20th century it was believed that the metaphysical justification for Occam's Razor was simplicity. It was thought that nature was in some sense simple and that our theories about nature should reflect that simplicity. With such a metaphysical justification came the implication that Occam's Razor is a metaphysical principle. From the beginning of the 20th century, these views fell out of favor as scientists presented an increasingly complex world view. In response, philosophers turned away from metaphysical justifications for Occam's Razor to epistemological ones including inductive, pragmatic, likelihood and probabilistic justifications, which is where things stand today. Thus, Occam's Razor is currently conceived of as a methodological principle. Elliott Sober has expressed dissatisfaction with epistemological justifications for Occam's Razor. He thinks that there must be a metaphysical presupposition for Occam's Razor, but offers no possibilities (Sober, 1990). For a summary of epistemological justifications for Occam's Razor see Roger Ariew's dissertation of 1976 "Ockham's Razor: A Historical and Philosophical Analysis of Ockham's Principle of Parsimony".
Occam's Razor has become a basic perspective for those who follow the scientific method. It is important to note that it is a heuristic argument that does not necessarily give correct answers; it is a loose guide to choosing the scientific hypothesis which (currently) contains the least number of unproven assumptions. Often, several hypotheses are equally "simple" and Occam's Razor does not express any preference in such cases.
For example, after a storm you notice that a tree has fallen. Based on the evidence of "a storm" and "a fallen tree" a reasonable hypothesis would be that "the storm blew down the tree" -- a hypothesis that requires only one assumption -- that it was, in fact, a strong wind that knocked over the tree, rather than a meteor or an elephant. The hypothesis that "the tree was knocked over by marauding 200 meter tall space aliens" requires several additional assumptions (concerning the very existence of aliens, their ability and desire to travel interstellar distances and the alien biology that allows them to be 200 meters tall in terrestrial gravity) and is therefore less preferable.
Occam's Razor is not equivalent to the idea that "perfection is simplicity". Albert Einstein probably had this in mind when he wrote in 1933 that "The supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience" often paraphrased as "Theories should be as simple as possible, but no simpler." It often happens that the best explanation is much more complicated than the simplest possible explanation because it requires fewer assumptions. Some people have oversimplified Occam's Razor as "The simplest explanation is the best (or true) one".
There are two senses in which Occam's razor can been seen at work in the history of science. One is ontological reduction by elimination and the other is by intertheoretic competition. In the former case the following are examples of reduction by elimination: The impetus of Aristotelian Physics, the angelic motors of medieval celestial mechanics, the four humors of ancient and medieval medicine, demonic possession as an explanation of mental illness, Phlogiston from premodern chemistry, and vital spirits of premodern Biology.
In the latter case there are three examples from the history of science where the simpler of two competing theories each of which explains all the observed phenomena has been chosen over its ontologically bloated competitor: the Copernican heliocentric model of celestial mechanics over the Ptolemaic geocentric model, the mechanical theory of heat over the Caloric theory, and the Einsteinian theory of electromagnetism over the luminiferous aether theory. In the first example, the Copernican model is said to have been chosen over the Ptolemaic due to its greater simplicity. The Ptolemaic model, in order to explain the apparent retrograde motion of Mercury relative to Venus, posited the existence of epicycles within the orbit of Mercury. The Copernican model (as expanded by Kepler) was able to account for this motion by displacing the Earth from the center of the solar system and replacing it with the sun as the orbital focus of planetary motions while simultaneously replacing the circular orbits of the Ptolemaic model with elliptical ones. In addition the Copernican model excluded any mention of the crystaline spheres that the planets were thought to be embedded in according the Ptolemaic model. In a single stroke the Copernican model reduced by a factor of two the ontology of Astronomy. According to the Caloric theory of heat, heat is a weightless substance that can travel from one object to another. This theory arose from the study of cannon boring and the invention of the steam engine. It was while studying cannon boring that Count Rumford made observations that conflicted with the Caloric theory and he formulated his mechanical theory to replace it. The Mechanical theory eliminated the Caloric and was thus ontologically simpler than its predecessor. During the 19th century Physicists believed that light required a medium of transmission much as sound waves do. It was hypothesized that a universal aether was such a medium and much effort was expended to detect it. In one of the most famous negative experiments in the history of science, the Michelson-Morley experiment failed to find any evidence of its existence. Einstein capitalized on this finding and constructed his theory without any reference to the Aether, thus providing another example of a theory chosen in part for its greater ontological simplicity.
Biologists or philosophers of biology use Occam's Razor in either of two contexts both in evolutionary biology: the units of selection controversy and Systematics. George C. Williams in his book Adaptation and Natural Selection (1966) argues that the best way to explain altruism among animals is based on low level, i.e. individual, selection as opposed to high level group selection. Altruism is defined as behavior that is beneficial to the group but not to the individual, and group selection is thought by some to be the evolutionary mechanism that selects for altruistic traits. Others posit individual selection as the mechanism which explains altruism solely in terms of the behaviors of individual organisms acting in their own self interest without regard to the group. The basis for Williams' contention is that of the two, individual selection is the more parsimonious theory. In doing so he is invoking a variant of Occam's Razor known as Lloyd Morgan's Cannon.
However, more recent work by biologists have revealed that Williams' view is not the simplest and most basic. The way evolution works is that the genes that are propagated in most copies will end up determining the development of that particular species, i.e., natural selection acts to select specific genes, and this is really the fundamental underlying principle, that automatically gives individual and group selection as emergent features of evolution.
However, a much simpler explanation immediately offers itself, once you realise that natural selection works on genes. If the male musk ox runs off, leaving his offspring to the wolves, his genes will not be propagated. If however he takes up the fight his genes will live on in his offspring. And thus the "stay-and-fight" gene prevails. This an example of kin selection. An underlying general principle thus offers a much simpler explanation, without retreating to special principles as group selection.
Systematics is the branch of biology that attempts to establish genealogical relationships among organisms. It is also concerned with their classification. There are three primary camps in systematics; cladists, pheneticists, and evolutionary taxonomists. The cladists hold that genealogy alone should determine classification and pheneticists contend that similarity over propinquity of descent is the determining criteria while evolutionary taxonomists claim that both genealogy and similarity count in classification.It is among the cladists that Occam's razor is to be found, although their term for it is cladistic parsimony. Cladistic parsimony (or maximum parsimony) is a method of phylogenetic inference in the construction of cladograms. Cladograms are branching tree like structures used to represent lines of descent based on one or more evolutionary change(s). Cladistic parsimony is used to support the hypothesis(es) that require the fewest evolutionary changes. It should be noted that for some types of tree, it will consistently produce the wrong results regardless of how much data is collected (This is called long-branch attraction ). For a full treatment of cladistic parsimony see Elliott Sober's Reconstructing the Past: Parsimony, Evolution, and Inference (1988). For a discussion of both uses of Occam's Razor in Biology see Elliott Sober's article Let's Razor Occam's Razor (1990).
When discussing Occam's Razor in contemporary medicine, doctors and philosophers of medicine speak of diagnostic parsimony. Diagnostic parsimony advocates that when diagnosing a given injury, ailment, illness, or disease a doctor should strive to look for the fewest possible causes that will account for all the symptoms.
Probably the first person to make use of the principle was Ockham himself. He writes "The source of many errors in philosophy is the claim that a distinct signified thing always corresponds to a distinct word in such a way that there are as many distinct entities being signified as there are distinct names or words doing the signifying. (Summula Philosophiae Naturalis III, chap. 7, see also Summa Totus Logicae Bk I, C.51). We are apt to suppose that a word like "paternity" signifies some "distinct entity", because we suppose that each distinct word signifies a distinct entity. This leads to all sorts of absurdities, such as "a column is to the right by to-the-rightness", "God is creating by creation, is good by goodness, is just by justice, is powerful by power", "an accident inheres by inherence", "a subject is subjected by subjection", "a suitable thing is suitable by suitability", "a chimera is nothing by nothingness", "a blind thing is blind by blindness", " a body is mobile by mobility". We should say instead that a man is a father because he has a son (Summa C.51).
Another application of the principle is to be found in the work of George Berkeley(1685-1753). Berkeley was an idealist believing that all of reality could be explained in terms of the mind alone. He famously invoked Occam's Razor against Idealism's metaphysical competitor materialism claiming that matter was not required by his metaphysic and was thus eliminable. Idealism has few adherents today and Berkeley's arguments find few sympathetic ears.
In the 20th century Philosophy of Mind, Occam's Razor found a champion in J.J.C. Smart who in his article "Sensations and Brain Processes"(1959) claimed Occam's Razor as the basis for his preference of the mind-brain identity theory over mind body dualism. Dualists claim that there are two kinds of substances in the universe: physical (including the body) and mental, which is nonphysical. In contrast identity theorists claim that everything is physical, including consciousness, and that there is nothing nonphysical. The basis for the materialist claim is that of the two competing theories, dualism and mind-brain identity, the identity theory is the simpler since it commits to fewer entities. Smart was criticized for his use of the razor and ultimately retracted his advocacy of it in this context.
Paul Churchland (1984) cites Occam's Razor as the first line of attack against dualism but that by itself it is inconclusive. The deciding factor for Churchland is the greater explanatory prowess of a materialist position in the Philosophy of Mind as informed by findings in Neurobiology.
Dale Jacquette (1994) claims that Occam's Razor is the rationale behind eliminativism and reductionism in the Philosophy of Mind. Eliminativism is the thesis that the ontology of folk Psychology including such entities as "pain", "joy", "desire", "fear" etc. are eliminable in favor of an ontology of a completed neuroscience.
In the philosophy of religion Occam's Razor is sometimes used to challenge arguments for the existence of God. None of these applications has been considered definitive because the competing assumptions are not (and perhaps cannot be) precisely defined. Also, it should be added that the principle is only a guide to the best theory based on current knowledge, not the "truth."
William may have been inspired by earlier thinkers. For example, Book V of Aristotle's Physics has the statement "Nature operates in the shortest way possible."
Galileo Galilei lampooned the misuse of Occam's Razor in his Dialogue. The principle is represented in the dialogue by Simplicio.The telling point that Galileo presented ironically was that if you really wanted to start from a small number of entities, you could always consider the letters of the alphabet as the fundamental entities, since you could certainly construct the whole of human knowledge out of them. (A view that Abraham Abulafia held much more expansively.)
Adding another layer of irony, many modern scientists and mathematicians seriously propose that the basic "entities" of reality may be "bits of information", i.e. the digits of binary code, in which case the entities of William of Occam might be seen as foreshadowing the logic of George Boole and modern computing.
Perhaps due to the abstruse nature of medieval logic and the obscure goals of William of Occam as a theologian and logician, discussion and application of Ockham's Razor is frequently full of ironies.
It is argued that Ockham was an intellectual forefather of the Scientific Method because he argued for a degree of intellectual freedom in a time of dogmatic belief. He can also, however, be seen as an apologist for Divine Omnipotence, since he was concerned to demonstrate that creation was contingent and the Creator free to change the rules at will. Thus, if God is free to make an infinity of worlds with completely different rules from those which prevail in our world, then we are free to imagine such worlds and their logical and practical consequences (within the bounds set by the Church's infallible Dogma).
Perhaps the best formulation of Occam's Razor is the one which states that, of equally good explanations for a phenomenon, the best one is the simplest explanation which accounts for all the facts.
Occam's Razor could also be said to apply to the elimination of the soul as a superfluous entity.
Creationists sometimes employ Occam's Razor in defense of their theory of Cosmogenesis and Geogenesis claiming that theirs is the simpler theory as compared with evolution. However, because this is an attempt to apply Occam's Razor to the past, where observation to verify or falsify its use is not possible, this use of Occam's Razor is disputed. The disputation continues on question of whether the postulation of a Creator with cogniscence and forethought is "simpler" than one in which complexity spontaneously generates over geological timescales.
There are various papers in scholarly journals deriving versions of Occam's Razor from probability theory and applying it in statistical inference, and also of various criteria for penalizing complexity in statistical inference. Recent papers have suggested a connection between Occam's Razor and Kolmogorov complexity.
One of the problems with the original formulation of the principle is that it only applies to models with the same explanatory power (i.e. prefer the simplest of equally good models). A more general form of Occam's Razor can be derived from Bayesian model comparison and Bayes factors, which can be used to compare models that don't fit the data equally well. These methods can sometimes optimally balance the complexity and power of a model.
Many artificial intelligence researchers are now employing strongly probabilistic Bayesian techniques.
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