But even if this be admitted, it does not follow that Euclid means the same thing in his definition of a straight line; all the evidence, it seems to me, points in another direction.

This command is used to define new lines or to redefine existing lines. If an existing line is redefined then the position of any existing parts whose definition depends on the line will change ...

The symmetry can be tested by turning the line over; for in stance, the edge of a flat ruler is straight if, when turned over, it coincides with its original position.