O give the Reader an account, Why the following Treatise is suffer’d to pass abroad so maim’d and imperfect, I must inform him that ’tis now long since, that to gratify an ingenious Gentleman, I set down some of the Reasons that kept me from fully acquiescing either in the Peripatetical, or in the Chymical Doctrine, of the Material Principles of mixt Bodies. This Discourse some years after falling into the hands of some Learned men, had the good luck to be so favourably receiv’d, and advantageously spoken of by them, that having had more then ordinary Invitations given me to make it publick, I thought fit to review it, that I might retrench some things that seem’d not so fit to be shewn to every Reader, And substitute some of those other things that occurr’d to me of the trials and observations I had since made. What became of my papers, I elsewhere mention in a Preface where I complain of it: But since I writ That, I found many sheets that belong’d to the subjects I am now about to discourse of. Wherefore seeing that I had then in my hands as much of the first Dialogue as was requisite to state the Case, and serve for an Introduction as well to the conference betwixt Carneades and Eleutherius, *as to some other Dialogues, which for certain reasons are not now herewith publish’d, I resolv’d to supply, as well as I could, the Contents of a Paper