Thursday, October 11, 2007

Possible, Plausible, Probable, Proven

I wrote this post for the blog I write for a course, Peoples and Cultures of the World, and originally intended it primarily for a student audience. However, I think it fits well here as well.

“Possible,” “Plausible,” “Probable,” and “Proven” are terms used to indicate rough degrees of statistical probability of something happening or some proposition being true. (My use of the “probable” here reflects the vernacular. When we say that something is probably true, we don’t mean that it has just any level of statistical probability, but specifically that it is quite likely to be true.)

The terms do reflect an ascending order of probability (and a nested one – anything that is plausible is also possible; anything proven is also probable, plausible, and possible), though not in a numerically precise way. They represent a sort of qualitative statistics. When we can realistically indicate precise probabilities, that is obviously a useful thing, but even a rough sense of degree of probability is far more useful than no such sense at all.

Errors in thinking arise whenever we jump up this ascending ladder of probability without evidence, or without sufficient evidence (though admittedly, knowing what counts as sufficient evidence is always tricky business). Just because it’s possible that Bigfoot could be running around the Pacific Northwest or elsewhere doesn’t make it plausible, much less probable or proven.

The Possible

Saying that something is possible simply means that it does not violate the basic laws of logic. In the realm of empirical scholarship, one could also add that it does not violate basic physical laws, that something is both logically and physically possible.

The existence of Bigfoot is possible – it violates no logical or physical rules, but given the overwhelming lack of evidence, there’s no reason to regard Bigfoot’s existence as having anything but the lowest degree of probability. The same goes for claims about extraterrestrial influence in building the Egyptian Pyramids or Stonehenge or the Nazca Lines.

The Plausible

To say that something is plausible is to indicate that it has a higher probability than the merely possible - it is believable, it makes sense. But claims that are merely plausible (that is, that are not also probable) lack the evidence to be taken as having a high degree of probability of truth.

Thor Heyerdahl’s famous voyage on his Kon-Tiki raft from South America to Polynesia certainly proved that it was possible for people to have traveled from the one place to the other using fairly simple watercraft. He even made it plausible that Polynesians could have made voyages to South America, but his voyage alone did nothing to make such notions probable, much less proven. (See this news article from this past summer from Live Science on both Heyerdahl and more recent evidence of Polynesian voyaging to South America that I’ll discuss below.)

An article I encountered this morning on Science Daily, “Early Apes Walked Upright 15 Million Years Earlier Than Previously Thought, Evolutionary Biologist Argues,” makes what I’d consider a plausible claim. “An extraordinary advance in human origins research reveals evidence of the emergence of the upright human body plan over 15 million years earlier than most experts have believed. More dramatically, the study confirms preliminary evidence that many early hominoid apes were most likely upright bipedal walkers sharing the basic body form of modern humans.” So long as there’s evidence, it’s plausible that hominoid bipedalism might be much older than previously thought, but this is an extraordinary claim, and as such requires not simply a single study with good evidence, but a body of good evidence in order to be taken as probable, much less proven by many scholars.

The Probable and the Proven

To say that something is probable means that it is very likely to be the case, that it has a high degree of probability. To refer to something as proven implies that a claim is definitely true, though given the ever present possibilities of faulty observation (even systematic faulty observation), partial understanding or misunderstanding of empirical materials, nothing (at least outside the abstract realm of pure logic and mathematics) is ever demonstrated to be completely and irrevocably true. Instead, to say something is proven is really to say that it has such a high degree of probability of truth that we can pragmatically assume it to be true (though ideally keeping an open mind towards potential counter-evidence).

When Pizarro and his Spanish soldiers reached Peru, they encountered chickens (an Old World domesticated bird) already there. There are at least a couple ways the chickens could have arrived in the New World – they could have been brought by the very earliest European voyages to the Caribbean and Central America in the 1490s and 1500s and very rapidly diffused southward; or they could have been brought by Polynesian voyagers to South America (the only problem there being, at least until now, a lack of evidence of such Polynesian voyages having actually occurred).

When Captain Cook and other explorers encountered a variety of Polynesian islands in the late 18th century, they encountered sweet potatoes, among other crops being grown. As I understand it, there’s no definite evidence of how these South American plants reached Polynesia. They could have been brought by the Spanish to the Philippines early in the Colonial period and diffused from there to Indonesia, Melanesia, and ultimately Polynesia, or they could have been brought back from South America by Polynesians themselves.

New evidence released this past summer addresses this situation. Chicken bones were recovered in Peru that, according to carbon dating, predate Spanish voyages to the Americas by about a century. Further, genetic evidence links the chicken bones to Polynesian varieties of chickens. (See the previously cited article from Live Science and also this article from New Scientist.)

If the carbon dating and DNA evidence hold up (always an important consideration with important new claims), this proves that Polynesian chickens reached Peru at least on one occasion. Given the highly implausible nature of chickens making the voyage on their own (though not logically impossible), it makes highly probable if not proving claims that Polynesians came to South America on at least one occasion. It makes highly probable that the chickens seen by Pizarro were of Polynesian stock as well. I’d even go so far as to say that this new evidence makes probable the idea that Polynesians brought sweet potatoes back from South America directly, though the distinction between plausible and probable is a bit more ambiguous in this case.


David Thole said...

Thanks for the interesting post. Here is a somewhat related article by Clive Thompson on the word "theory" as it relates to the Creation/Evolution debate.

Robert Philen said...

Thanks for the link to the article. I'd also recommend it to anyone interested either in the specific debate about evolution or the disjunctive way in which words like "theory" tend to be used in scientific and vernacular discourses.

Here's a selection from the article:

"If the antievolutionists insist on exploiting the public's misunderstanding of words like theory and believe, then we shouldn't fight it. "We need to be a bit less cautious in public when we're talking about scientific conclusions that are generally agreed upon," Quinn says.

"What does she suggest? For truly solid-gold, well-established science, let's stop using the word theory entirely. Instead, let's revive much more venerable language and refer to such knowledge as "law." As with Newton's law of gravity, people intuitively understand that a law is a rule that holds true and must be obeyed. The word law conveys precisely the same sense of authority with the public as theory does with scientists, but without the linguistic baggage.

"Evolution is supersolid. We even base the vaccine industry on it: When we troop into the doctor's office each winter to get a flu shot — an inoculation against the latest evolved strains of the disease — we're treating evolution as a law. So why not just say "the law of evolution"?

"Best of all, it performs a neat bit of linguistic jujitsu. If someone says, "I don't believe in the theory of evolution," they may sound fairly reasonable. But if someone announces, "I don't believe in the law of evolution," they sound insane. It's tantamount to saying, "I don't believe in the law of gravity."

"It's time to realize that we're simply never going to school enough of the public in the precise scientific meaning of particular words. We're never going to fully communicate what's beautiful and noble about scientific caution and rigor. Public discourse is inevitably political, so we need to talk about science in a way that wins the political battle — in no uncertain terms.

"At least, that's my theory."

Robert said...

I am interested by your explanation of the move from possible to plausible to probable to proven.

I was recently attempting to make a specific determination on which word to use in an assignment for my students. When I wrote the assignment (an argumentative essay about the two pieces of speculative fiction) I used the word "plausible." My students were unfamiliar with the term so I provided the definition from Webster's (which was wholly unsatisfactory). We then begin to research changing the assignment's wording to probable or possible. It was then that we encountered a conundrum.

We begin working through this issue from the perspective of certainty and were attempting to order the words according to most to least certain. Initially we listed the words (from most certain to least certain) as: probable, possible, plausible. We then reordered them as possible, probable, plausible. The real issue came at the point that we moved to their negatives and determined that as far as certainty is concerned the order must go like this (from most certain to least certain): impossible, improbable, implausible.

So, out of these three terms, is probable the most certain of the three terms when stated positively and impossible the most certain of the three when used negatively?

I have even gone to the OED to solve this dilemma, but the OED's definition of possible presents problems in that it states that is is something that can or may be true, etc. (introducing uncertainty into a term like impossible that specifically seems to negate any uncertainty). The OED's definitions for probable and plausible are inconclusive too.

What seems to be in the background at some point is the fact that the movement from possible to plausible to probable is not one of degrees, but of types and categories (and epistemology?) rather than of a simple progression such as small, smaller, smallest.

Any thoughts?

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Daniel Egger said...

@Robert I like your reflection. Do you have any additional materials, thinking about it. Your reflection of a as I interpret it for my reality, that the underlying logic of interpretation changes, also does the relationship of probable and plausible. Some may argue that the probable must fell under the defined plausible, but as logics shift and all depends on our perception and processing of information this must not be true.