75

O.64

It is verily not reasonable then to doubt that man has an aptitude for guessing some kinds of truth, as wonderful as the instincts of the lower animals; and it is often accompanied by an unaccountable and quite peculiar quality of confidence. It is impossible in purely geometrical terms to define a straight line: we can only say that there is a certain family of surfaces, the planes, any three of which that do not all contain a line in common have one and but one common point; and the intersection of any two of them is a ray, or unlimited straight line. But