Since metaphysics more than any other science ensures the clarity, precision, and scope of the mind, it is also the best preparation for the study of all the other sciences. It is today so little regarded in France that many readers will no doubt find my claim paradoxical. I confess that there was a time when I would have shared that opinion. Of all the philosophers, the metaphysicians seemed the least wise: their words did not tell me anything, hardly ever did I find anything but airy speculations, and I charged metaphysics with gross errors that were in fact the aberrations of its practitioners. Wishing to overcome this illusion and to find the source of so many errors, I found those who were farthest from the truth to be the most useful. I had scarcely recognized the uncertain paths they had followed before I thought I saw the way that I should follow. It appeared to me that one could reason in metaphysics and in the moral sciences with as much precision as in geometry, that we could form accurate ideas as well as the geometricians, like them determine the sense of expressions in a precise and invariable manner, and perhaps better than they have done prescribe a simple and easy procedure to attain certain knowledge.

We must distinguish two sorts of metaphysics. One has the ambition of solving all mysteries; nature, the essence of all beings, the most hidden causes, those are the things that embellish it and that it promises to open up.