"As earth is an element... it deserves an accurate investigation to discover which is the most simple and elementary of all the substances to which the name earth has been applied. ...considering, first, what are the essential properties by which earthy substances differ from other elements, and then by determining that earth to be the most pure and simple, which possesses these properties most eminently and decisively; for ...the more eminently any substance possesses these characteristic properties... the nearer it approaches to this element..."