What right have we to make such an assumption as this?

The question was first clearly put by Hume, and was handled by him with
singular lucidity; but his answer, though very near the truth, was not
so expressed as to set the question at rest.

The main relation in which the uniformity of Nature is observed is that
of cause and effect. Hume examines this and maintains that there is
absolutely nothing contained in it but the notion of invariable
sequence. Two phenomena are invariably found connected together; the
prior is spoken of as the cause, the posterior as the effect. But there
is absolutely nothing in the former to define its relation to the
latter, except that when the former is observed the latter, as far as we
know, invariably follows. A ball hits another ball of equal size, both
being free to move. There is nothing by which prior to experience we
can determine what will happen next. It is just as conceivable that the
moving ball should come back or should come to rest, as that the ball
hitherto at rest should begin to move. A magnet fastened to a piece of
wood is floating on water. Another magnet held in the hand is brought
very near one of its poles or ends. If two north poles are thus brought
together the floating magnet is repelled; if a north and a south pole
are brought together the floating magnet is attracted. The motion of the
floating magnet is in each case called the effect; the approach of the
magnet held in the hand is called the cause. And this cause is, as far
as we know, invariably followed by this effect. But to say that one is
cause and the other effect is merely to say that one is always followed
by the other; and no other meaning, according to Hume, can be attached
to the words cause and effect.