Everyone has it

We consider common sense as a philosophical principle.

Montaigne famously observed that sense, or common sense, was the most fairly distributed gift among people, in that everyone is satisfied with the share he or she has.  His observation is normally quoted in order to show how mistaken people can be, since others often judge a particular person’s common sense as inadequate.  (Montaigne’s own reasoning was more subtle.)  It came as something of a surprise to our tutor, then, to read a very similar observation made by Descartes, and made the very basis of his philosophy.  That is, he held that each person had a certain natural sense, that if employed correctly would enable him or her to distinguish the true from the false; and on this Descartes built his whole structure.

These different interpretations of the observation might be connected with the situations of each of these Frenchmen.  Montaigne lived through the French Wars of Religion, in which the opposing sides each claimed to have the ultimate truth, though the truths were contradictory.  Descartes’ France was the more unified and organized one of Richelieu, even if the philosopher found it expedient to live in Holland.  There was also the influence of the Jesuits; but going into that would be a large digression.

Descartes’ ideal for truth is mathematics or geometry.  He repeatedly cites, as an example of something that can be proved as true by common sense or natural reason, the fact that the three angles of a triangle add up to two right angles.  Certainly the rigor of mathematics is appealing to someone trying to sort out the various schools of philosophy, or someone unhappy with the sterile disputes of scholasticism.  The trouble is that Descartes’ ideal isn’t true.

The two-right-angles conclusion is based on an assumption, a postulate, that geometers had been unhappy with since Euclid introduced it in ancient times.  For two thousand years they had tried to prove it from the other postulates, without success.  Then, in the nineteenth century, Boylai and Lobachevsky (anticipated privately by Gauss) asked: what if it isn’t true?  Well then, you get a non-Euclidean geometry that looks strange and unfamiliar, but is just as consistent as what everyone was used to.  After much development, it formed part of the basis of General Relativity, the very successful theory of gravity of today.  Common sense had failed.

With the twentieth century came Quantum Mechanics, the (also successful) theory of how things work on tiny scales.  It is completely at odds with common sense.  Our natural ideas of how things work, based on normal life, are no guide to the very fast, the very small or the very heavy.

There is a cautionary tale here.  But common sense will reject it.