The Role of Perception in Science: Part 5 “Logical Conjecture”

Testing Assumptions around a Long-Established Model
In the 1930s, the Land Bridge Theory was the best available explanation for the settlement of North America. Although the model had merit it still lacked the ability to be effectively tested. Models incapable of either being confirmed or falsified have more in common with speculation (or magical thinking) than with genuine science. Effective models enable scientists to make predictions; and if a prediction is confirmed it suggests the model is, at the very least, approximately true; and over time the more evidence acquired in support of a model increases the trustworthiness of that model.

Concrete evidence in support of the Land Bridge Theory came in 1949 when chemist Willard Libby introduced the world to radiocarbon dating. Radiocarbon dating was the first reliable technique for determining the absolute age of carbon-based (organic) objects. After radiocarbon tests were conducted on archaeological sites throughout North America, it was established that the oldest sites of human settlement were on the West Coast (just as the model predicted).

Scientists predicted the earliest settlements were located on the West Coast because the climate there was comparatively more hospitable than that of the interior. Predictions like these are developed in part through evidence gathering and logical conjecture, e.g. no people would settle, let alone survive, in too harsh a climate (especially when better options existed on the coast).

Another piece of evidence favoring a coastal settlement hypothesis was the fact that during the most recent ice age North America’s interior was not only too cold to support people but also unreachable because of glaciers. By contrast people living in coastal settlements could take advantage of both the abundance of fish/shellfish but also a climate made more livable by the influence of warm ocean air. Scientists conjectured that the interior was gradually populated with the recession of the glaciers about 11,700 years ago.

If the Land Bridge Theory had a weakness it was the assumption the Western Hemisphere was only ever settled by humans once in the planet’s entire history. If there was only one migration event, logically-speaking, Native Americans currently living in North America would be genetically similar to people living in Siberia (the assumed source of the first initial migration to North America 13,000 years ago). In the 1970s, improvements in our understanding of genetics allowed us to finally test this prediction.

Test results revealed Siberians and North America’s indigenous peoples did in fact belong to the same haplogroup (and therefore shared a common ancestor in the not too distant past). The one migration hypothesis seemed to be getting stronger and stronger with every subsequent piece of evidence—logical, archaeological or genetic.

Logical Conjecture & Theory Construction
In his book The Grand Design, theoretical physicist Stephen Hawking observed scientists prefer building theories that are simple and elegant over ones which are un-necessarily complex. At first glance the Land Bridge Theory seems to fit Hawking’s requirements—it is simple, straightforward, and supported by multiple lines of evidence. So what could possibly be wrong with such a theory?

Well, a theory is ultimately only as strong as the evidence in support of it; and since scientists do not know everything they frequently have to make (logical) assumptions; and if experts do not have all the information the assumptions they make might lead them to make mistakes or miss important details. Despite the problem of not having all the information the scientific method is still nonetheless a powerful tool of discovery. This is because when new evidence emerges we improve our theories by changing them to take in to account the new information.

Some critics of science point out that since we keep making mistakes and scientific knowledge is continually changing science is inherently untrustworthy. People who hold such a view do not understand the scientific method very well; that is, in many respects science works on the basis of trial and error, e.g. a scientist tries one approach and it doesn’t work so they try another, etc. In the end, when scientists admit they’ve made mistakes they use what they’ve learned to refine their theories making them more accurate (which most sane people would admit is a good thing). But to refine a theory you need more and better information; you need to confirm assumptions.

Logical conjecture is used whenever experts need to reach a tentative conclusion. In other words conjecture acts as a temporary stepping stone towards a better understanding of things. Logical conjecture is not quite the same thing as an educated guess. This is because when such conjecture is used no guessing is attempted whatsoever. Rather, experts make use of the available evidence (you need a concrete starting point) to make predictions of what probably happened, what is probably happening, or what might happen.

The Power of Logical Conjecture: Hubble & Lemaître
Conjecture is a powerful tool: for example, while walking in an alley you notice a broken window in the apartment building to your right. You see shards of glass and a baseball on the ground. If you were just guessing at what broke the window you’d resort to a claim not supported by the available evidence, e.g. powerful sound waves broke the window. Though it isn’t impossible sound was responsible for the damage, it is not supported by the available evidence (the source of such a sound is absent). Also, invoking Ockham’s Razor (or the notion the “simplest explanation is the most likely”) we can assume the cause of the broken glass is probably not so dramatic.

When using logical conjecture you do not guess: you begin with what evidence you have (a ball, shards of glass nearby) and move towards making a more reasonable conclusion like the ball was thrown probably breaking the window. This explanation, compared to the sound hypothesis, is simpler and more likely. But is it the only possible explanation? No. Yet, it appears to be the only one supported by the available evidence; and we use this information to construct a predictive model.

Logical conjecture is useful for helping us organize small pieces of evidence in to a ‘bigger picture.” Specifically, it allows us to complete a puzzle even though many pieces of the puzzle might be missing. For example, in 1924 astrophysicist Edwin Hubble (1889-1953) observed that some galaxies appeared to be “blue-ish” while others appeared “red-ish.” Hubble was initially confused by what he observed; however, he knew that both light and sound traveled as waves. In the case of sound, waves sound different depending upon whether or not sound is moving away from as opposed to towards an observer. This well-known phenomenon is called the Doppler Effect. Hubble used his understanding of sound and extrapolated that light also behaved as though it too expressed a Doppler Effect; that is, objects moving away from an observer cast red light while objects moving towards an observer cast blue. Physicists call this confirmed phenomenon “red shift.”


In 1927, astronomer Henri Joseph Édouard Lemaître (1894-1966) built on the work of Hubble. He conjectured that since galaxies were moving away from one another that at some time in the distant past star systems must have been closer together, even occupying a single point in space (13.82 billion years ago according to the most current work on the topic). To put it another way: the fact galaxies were moving away from us implied space was expanding (which likewise implied the prior existence of a central starting point or singularity).

The idea of an expanding universe is an idea most of us find reasonable or take for granted today. Yet, prior to Lemaître’s work the vast majority of astrophysicists (including Albert Einstein) believed the universe was in fact static, immovable, set in place “as is.” (Most physicists also figured the universe did not have a distinct beginning but rather simply “always was.”)

Lemaître’s work led to the eventual development of the Big Bang Theory. The Big Bang Theory is all fine and good, but what does it have to do with our discussion regarding logical conjecture? Well, we cannot actually go back in to the past and see the Big Bang occurring. Therefore, we use the available evidence—red shifted galaxies—deducing all matter at some point in the distant past occupied a single point, eventually exploding in to the expanding universe we observe today. (Stephen Hawking’s work with black holes in the 1960s actually established a theoretical framework for the universe not only having a definite beginning but that the universe emerged out of literally “nothing.”) These conclusions were not guessed at; they were the products of a disciplined use of logic and evidence.

The Necessity of Gathering More Evidence
No method of reasoning or evidence gathering is perfect: a theory is only as good as the evidence acquired in support of it. If experts are missing critical pieces of information their models will not be accurate. Such theories might even lead you to make false assumptions about reality.

For example, let’s return our attention to the alley (see image below). When we formed our first hypothesis, e.g. the ball broke the window, etc. we weren’t aware of several other vital pieces of information. When we examine the scene more closely we discover that a cat was hiding behind a garbage can where we also find a broken pot and plant. This new evidence requires we return to our original hypothesis: specifically, our assumption that the ball broke the window is now not the only reasonable explanation for what happened. Based on this new information we would be justified in forming a second hypothesis, e.g. the cat knocked the plant through the window above. Which hypothesis is correct? Until both hypotheses are tested we don’t actually know which model to use (both models make useful predictions). Ultimately, hypotheses are only useful starting points to begin an investigation.


The biggest problem with logical conjecture is that in order to use it we have to make additional or secondary assumptions. To this end our understanding of particle physics has improved over the past century of study; however, we do not know everything there is to know about this field. Yet, we are still developing some pretty interesting technologies using the Standard Model of physics, e.g. GPS, space travel, satellites, etc. Nonetheless, some humility is in order; that is, when we are too confident in the explanatory power of a theory we risk blindness (failing to either improve our existing understanding or see how that understanding might be flawed).

The Problem with Inheriting Assumptions
Scientists, just like everyone else, inherit assumptions from their parent culture. The parent culture shapes how they look at the world. In the previous section, I mentioned how most people simply take it for granted that the Big Bang is a fact. Why do the majority of people accept it as true? Based on the evidence? Perhaps. Though to be honest I’m not so sure. The theory itself is simple enough to grasp but how many people are actually acquainted with the Big Bang’s supporting evidence like “red shift” or the “cosmic background radiation”? In my honest opinion, the theory has been around for so long, and is so well-established in the parent culture most of us just grow up believing (rather than understanding) the Big Bang is an accurate description of the universe’s origins. We are, after all, creatures with believing brains. With this in mind, let’s return our attention back to the Land Bridge Theory.

According to the earliest renditions of the Land Bridge Theory there was only ever one migration of people from Asia to North America. Only one. Even the evidence in our possession as recently as the 1990s seemed to support the one-migration view. But what are scholars saying about it in 2015? Can we still learn anything new about how the North, South and Central America were populated? I guarantee we have not searched every inch of land. There must still be important discoveries to make. Therefore, it seems to me to be incredibly likely we’ll discover archaeological sites older than 13,000 years of age (and, as coincidence would have it, we have).

Sometimes when we think we’ve answered a question once and for all, or when we stop asking meaningful questions, we grow too confident in our theories; and that confidence frequently gets us in to trouble; this is because when theories cease being accepted on the basis of evidence alone, and instead become something simply believed in, the affected scientist becomes less willing (or, worse still, less capable of conceiving of alternate explanations). For example, if I believe there was only one migration event then the evidence of a settlement (dated to 20,000 BCE) will either be missed or possibly ignored, i.e. expectation blinds us to the possibility that reality is different than what we were taught; and when we believe in theories too strongly bias makes us incapable of seeing the forest for the trees.

Successfully Challenging the Land Bridge Theory
If you took a history or social studies class in North America over the past several decades, it is likely you’ve been taught one or both of the following ideas:

1). Before the arrival of Columbus in 1492 AD the “New World” was an untamed wilderness.
2). The sparse populations who lived on this virtually empty land were unremarkable civilizations.

Now if I were taught (and believed) these two ideas accurately described the settlement process of North America, the one migration hypothesis becomes more plausible: nonetheless, practicing good science requires we do not become indefinitely beholden or believe in any one idea.

(For this very reason many physicists in England were reluctant to accept the validity of Einstein’s General Theory of Relativity when it was first proposed in 1915. This is because for three centuries Newtonian mechanics sufficiently explained and gave order to the universe. Thus, physicists ignored Einstein’s challenge until Arthur Eddington proved by observation one of relativity’s chief predictions in 1919, e.g. light bent due to gravity’s influence. Newton was effectively unseated. Einstein never receiving a Nobel Prize for relativity perhaps reflects in part the bitterness felt by some English members of the scientific community to see Isaac Newton’s position diminished by a German-Jewish scientist. Click here for more on Eddington’s experiment.)

We must be free, like Arthur Eddington, to question any authority—scientific or otherwise. We must go where the evidence takes us and it is wise to remain skeptical of even our best established scientific theories. The importance of skepticism cannot be over-stated: questioning established wisdom is the only way to grow in our knowledge or understanding of the world.

The Land Bridge Theory started coming under fire in 1997 when an archaeological dig in Southern Chile (see Chinchihuapi Creek at Monte Verde) revealed compelling evidence of human habitation much earlier than 13,000 thousand years ago. Scientists initially struggled to reconcile this new information with the existing Land Bridge Theory as it was first proposed. According to the original theory this ancient Chilean civilization should not have even existed… Yet, exist it did. Scientists have subsequently conjectured that the early settlers of Chile must have made the journey southward before the ice-free corridor ever appeared (if it ever appeared at all—new evidence has emerged even placing the existence of an ice-free corridor idea in to doubt); moreover, there appears to be a haplogroup of a people living in the interior of South America that does not match up with Siberian peoples; therefore, there must have been at least two separate settlements.

In his book 1491, science writer Charles C. Mann observed that “[given] the near impossibility of surpassing the glaciers without the corridor, some archaeologists suggested the first Americans must have arrived twenty thousand years ago, when the ice pack was smaller. Or even earlier than that—the Chilean site had suggestive evidence of artifacts more than thirty thousand years old. Or perhaps the first Indians traveled by boat, and didn’t need the land bridge. Or maybe they arrived via Australia, passing the South Pole” (Mann, Page 19).

Although confidence in the Land Bridge Theory has been shaken by new evidence the model itself has not been discarded. Despite the theory’s apparent shortcomings it nonetheless still has explanatory power, e.g. we can still use it to describe the most recent human migration event to North America (which will form part of a bigger, more encompassing theory in the future). For this reason the theory is changing to accommodate the new findings like the archaeological sites in Chile and elsewhere. Again, to quote Charles Mann, “Scientists do not necessarily agree on all of the details; some scientists have theorized that the Americas may have been hit with as many as five waves of settlement before Columbus, with the earliest occurring as much as fifty thousand years ago. In many versions, though, today’s Indians are seen [comparatively speaking] as relative latecomers” (19).

SOURCE: Mann, Charles. 1491: New Revelations of the Americas Before Columbus. New York: Vintage Books, 2005.



One thought on “The Role of Perception in Science: Part 5 “Logical Conjecture”

Comments are closed.