Saturday, 16 August 2014


Commonsense tells me that the Earth is flat, but if I climb to the top of a really high mountain, I will notice that the horizon has a slight curvature. Without straining my legs to climb the mountain, I notice that the stars all move in predictable patterns at night, along with a bunch of "round" elements in the sky hinting that maybe the Earth has a similar shape. In Ancient Egypt, Eratosthenes found that a pole cast no shadow on midday in Syene, south of where he lived, whereas at his home in Alexandria a pole orthogonal to the ground did cast one. On a flat Earth, this could happen if the sun were very close so that its rays did not arrive parallel to one another, but the phenomenon was better explained by hypothesizing a spherical Tellus. The great Eratosthenes used his common sense to figure out that the cosmic shores on which we live are those of a round planet, and even calculated its circumference.
More commonsense makes for interesting insights, which might seem counterintuitive to those using less of it. To me, it is self-evident that logic is the relentless use of commonsense defined in its ordinary sense as the methods of reasoning available to the common man. Trust in observation that is not gainsaid by other observation is also commonsense. Put the two together and commonsense is really all that one needs to learn anything that is learnable. If two propositions appear inconsistent, commonsense suggests that either at least one of them is wrong or that appearances deceive. More commonsense can find out which, just like Eratosthenes could have used his shadow observations to question his belief (had he ever thought so, which I doubt he did) that the Earth was flat.
Then why do people expounding "counterintuitive" results always appear to do it with a great deal of pride? By my reckoning, it must be because they have not grasped that commonsense rightfully occupies the throne in the realm of epistemology. Sure, something might seem counterintuitive, but once commonsense as ordinarily defined has been applied every bit of the way in reaching that "counterintuitive" answer, it will be found that the original "counterintuitive" conclusion was actually pretty intuitive, quite within the reach of commonsense (or it will be found that it was wrong or that one should actually suspend judgement).
If something is counterintuitive, it must mean that it "counters", or somehow opposes, "intuition", but if intuition is what we can grasp by use of commonsense (like the fact that Tellus is spherical), it should be regarded a cardinal sin of epistemology to praise anything that is truly counterintuitive. If a researcher has to describe his theses or results by use of such foul words, my guess is he probably has not practised his intuition as much as he should have. But that is not to criticize, because vanishingly few people practise enough.
The late Gary Becker once told a story about how he met someone on a journey by aeroplane, with whom my hero (i.e. Professor Becker) had a talk about research, among other things. The fellow passenger told Professor Becker that he meant "no offence, but your [Becker's] ideas seem like plain commonsense". Professor Becker told the story to illustrate that he thought great ideas in Economics are in fact commonsensical, and that he of course had taken no offence at all. The important thing to remember is that the world is a place full of intricate relationships and that commonsense cannot get very far if not applied consistently to dig deeper into the complex. Commonsense does not mean that the yokelry know all that can be known, only that they are capable of it.

(This post is an expansion upon my comment on a recent EconLog blog post.)

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