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families of lines of which there is one and one only through any two points.
It is a law of dynamics that every
dynamical relation between two points,
no third point being concerned, except by combinations
of such pairs, is altogether similar except in quantity to every
such dynamical relation between any other
two points on the same ray, or straight line. It is a consequence
of this that a ray or straight line is the
shortest distance between two points; whence, light
appears to move along such lines; and that
being the case, we recognize them by the eye,
and call them straight. Thus, the faculty of
sight naturally causes us to assign great
prominence to such lines; and thus when
we come to form a hypothesis about the motion