I fear that the chief Reason why Chymists have written so obscurely of their three Principles, may be, That not having Clear and Distinct Notions of … - Robert Boyle

I consider, that as generally as Chymists are wont to appeal to Experience, and as confidently as they use to instance the several substances separated by the Fire from a Mixt Body, as a sufficient proof of their being its component Elements: Yet those differing Substances are many of them farr enough from Elementary simplicity, and may be yet look'd upon as mixt Bodies, most of them also retaining, somewhat... of the Nature of those Concretes whence they were forc'd.

But I will not... trouble you with what I have largely discoursed in the Sceptical Chymist, to call in question the grounds on which Chymists assert, that all mixt bodies are compounded of Salt, Sulphur, and Mercury. For it may suffice me now to tell you that, whatsoever they may be able to obtain from other bodies, it does not appear by Experience, which is the grand, if not the onely, Argument they rely on, that all mixt bodies that have Qualities consist of their tria prima, since they have not been able, that we know, truly, and without new Compositions, to resolve into those three, either Gold, or Silver, or Crystal, or Venetian Talck, or some other bodies, that I elsewhere name; & yet these bodies are endowed with divers Qualities, as the two former with Fusibleness and Malleability, and all of them with Weight and Fixity; so that in these and the like bodies, whence Chymisats have not made it yet appear, that their Salt, Sulphur and Mercury, can be truly and adequately separated, 'twill scarce be other than precarious to derive the malleableness, colour, and other Qualities of such bodies from those Principles.

I remember, that being once at Leyden, I was brought to the top of a tower, where, in a darkened room (such as is now used in many places to bring in the species of external object) a convex glass, applied to the only hole, by which light was permitted to enter, did project upon a large white sheet of paper, held at a just distance from it, a lively representation of divers of the chief buildings in the town; all which, upon the admission of more light into the room, by opening the window, did immediately disappear, And methinks... that in divers of the philosophical theories, that have been formerly applauded, something not unlike this may be easily observed. for though, whilst they are looked on with such a weak and determinate degree of light, they may appear very artificial and well-proportioned fabrics, yet they appear so but in that twilight, as it were, which is requisite to their conspicuousness. For if but a full light of new experiments and observations be freely let in upon them, the beauty of those (delightful, but fantastical) structures does immediately vanish.