For it is evident philosophers would never have had recourse to such obscure and uncertain principles, had they met with any satisfaction in such as are clear and intelligible; especially in such an affair as this, which must be an object of the simplest understanding, if not of the senses.

Upon the whole, we may conclude, that it is impossible in any one instance to shew the principle, in which the force and agency of a cause is placed; and that the most refined and most vulgar understandings are equally at a loss in this particular.

If any one think proper to refute this assertion, he need not put himself to the trouble of inventing any long reasonings: but may at once shew us an instance of a cause, where we discover the power or operating principle.

This defiance we are obliged frequently to make use of, as being almost the only means of proving a negative in philosophy.
The small success, which has been met with in all the attempts to fix this power, has at last obliged philosophers to conclude, that the ultimate force and efficacy of nature is perfectly unknown to us, and that it is in vain we search for it in all the known qualities of matter.

In this opinion they are almost unanimous; and it is only in the inference they draw from it, that they discover any difference in their sentiments.