Here then are two things to be considered, viz.

the reasons which determine us to make the past a standard for the future, and the manner how we extract a single judgment from a contrariety of past events.
First we may observe, that the supposition, that the future resembles the past, is not founded on arguments of any kind, but is derived entirely from habit, by which we are determined to expect for the future the same train of objects, to which we have been accustomed.

This habit or determination to transfer the past to the future is full and perfect; and consequently the first impulse of the imagination in this species of reasoning is endowed with the same qualities.
But, secondly, when in considering past experiments we find them of a contrary nature, this determination, though full and perfect in itself, presents us with no steady object, but offers us a number of disagreeing images in a certain order and proportion.

The first impulse, therefore, is here broke into pieces, and diffuses itself over all those images, of which each partakes an equal share of that force and vivacity, that is derived from the impulse.

Any of these past events may again happen; and we judge, that when they do happen, they will be mixed in the same proportion as in the past.
If our intention, therefore, be to consider the proportions of contrary events in a great number of instances, the images presented by our past experience must remain in their FIRST FORM, and preserve their first proportions.