To the end that we may be convinced of that want of solidity which the greatest men have not known how to give to the proofs they have offered, but which they have successively imagined has established their positions, let us briefly examine what the most celebrated philosophers, what the most subtile metaphysicians have said. For this purpose we will begin with Descartes, the restorer of philosophy among the moderns, to whose sublime errors we are indebted for the effulgent truths of the Newtonian system. This great man himself tells us, "All the strength of argument which I have hitherto used to prove the existence of immaterial substances, consists in this, that I acknowledge it would not be possible, my nature was such as it is, that is to say, that I should have in me the idea of immateriality, if this incorporeity did not truly exist; this same immateriality, of which the idea is in me, possesses all those high perfections of which our mind can have some slight idea, without however being able to comprehend them." In another place he says, "We must necessarily conclude from this alone, that because I exist, and have the idea of immateriality, that is to say, of a most perfect being, the existence is therefore most evidently demonstrated." There are not, perhaps, many except Descartes himself, to whom this would appear quite so conclusive; who would be impressed with the conviction which he seems to imagine is so very substantive. _First_, We shall reply to Descartes, it is not a warrantable deduction, that because we have an idea of a thing, we must therefore conclude it exists; to give validity to such a mode of reasoning would be productive of the greatest mischief; would, in fact, tend to subvert all human institutions. Our imagination presents us with the idea of a sphinx, or of an hippogriff, besides a thousand other fantastical beings; are we, on that authority, to insist that these things really exist? Is the mere circumstance of our having an idea of various parts of nature, discrepantly jumbled together, without any other evidence as to the assemblage, a sufficient warrantry for calling upon mankind to accredit the existence of such heterogeneous masses? If a philosopher of the most consummate experience, of the greatest celebrity, one who enjoyed the confidence of mankind above every other, was to detail the faculties and perfections of these visionary beings, although he should hold them forth as the perfection of all natural combinations, would, I say, any reasonable being lend himself to the asseveration?