"I am not convinced carbohydrate-restricted diets meet the standards
of common sense, and thus, I am not convinced that more research is needed."
Dr. David Katz, New Haven Register, 14 April 2003

I couldn’t have said it better – or worse – myself. I despise faddy diets,
and so I’m right behind Dr Katz’s dismissal of the idea that severely limiting
the intake of carbohydrates is a healthy or sustainable way of losing weight.
(The fact that I adore pasta and pizza does not, of course, influence my judgement.)
And I’m also keen to find ammunition to support my cause, having discovered to my despair that a friend has followed Dr Atkins down his low-carb garden path.

I’d even go so far as to say that I’m very attracted to the dictates of common
sense. But that’s just where I’d be going wrong. For common sense is a poor
indicator of what is true, or for those who are suspicious of the "T"
word, of what is reliable, practical or efficacious.

This point was made forcibly in the biologist Lewis Wolpert’s book The Unnatural
Nature of Science. Wolpert points out that science repeatedly confounds
common sense, or at the very least what is seen as common sense at any particular
time. For example, common sense held that a heavy object would fall faster than
a light one. Common sense would say that if the world were a sphere, people
on the bottom half would fall off, and we’d definitely be propelled into space
if the world were spinning. Common sense – at least in the form of received
wisdom – says that going out into the cold in wet clothes increases your chances
of getting a cold. Common sense on all three counts is wrong.

It doesn’t take much to show that common sense is unreliable. Yet I too find
the lure of common sense almost irresistible. What I think we’re often doing
when we appeal to common sense is expressing a kind of exasperation. Something
seems obviously true or false to us and we really don’t think it is worth the
effort explaining why. Appealing to common sense is thus just a shorthand way
of saying we think something is obviously true or false. But it’s a misleading
strategy, since it evokes some kind of external authority. "Common sense"
implies a kind of universal standard of rationality which is more than just
what ordinary people happen to think, but which requires less than specialised
knowledge. But if it is more than just what people happen to think, doesn’t
it just become what we believe people ought to think? And if this is what we
mean when we talk about common sense, shouldn’t we say so and make clear why
we think they ought to agree with us? Saying it is just common sense is a way
of shifting responsibility for having to explain why we think what we do onto
some mythical judge of ordinary reason.

So common sense is not just unreliable, it’s hazy and ill-defined. We should
try to avoid using the phrase altogether and instead replace it with something
that at least makes the basis of our judgement clear: it is what people generally
think; it is what all experience points to being true; it is received wisdom;
it is what I think is obvious; it is self-evident. Not all of these are good
justifications for belief. But at least they are clear and make it possible
to honestly assess what the justifications are. Saying something is common sense
is just a way of trying to avoid justifying it altogether.