I am forced to append to this Preface a correction of one or two errors of absolutely vital importance, which were unfortunately overlooked in the text. On page 188, line 18, the date was intended to read 1643; on page 189, line 5, read _anno trigesimo tertio_ for _trigesimo anno_; and on line 6, _anno vigesimo tertio_ instead of _vigesimo anno_. But if these emendations restore the passage to its original integrity, a discovery which I have made while this work was passing through the press has entirely cancelled its value. I have been gratified with a sight of the original edition of Philalethes’ _Introitus Apertus_--a small octavo pamphlet in the original paper cover as it was published at Amsterdam in the year 1667. It definitely establishes that its mysterious author was born in or about the year 1623, or two years later than the Welsh adept, Thomas Vaughan, with whom he has so long been identified. This original edition is excessively scarce; I believe I am the only English mystic who has seen it during the present generation. The reader must please understand that the calculation in the pages referred to was based on the date 1643; this date, in the light of the original edition, has proved erroneous, and by a curious chance, that which was accidentally printed, turns out to be correct at the expense of the calculation. ANALYSIS OF CONTENTS. PAGE PREFACE 5 INTRODUCTORY ESSAY ON THE TRUE PRINCIPLES AND NATURE OF THE MAGNUM OPUS, AND ON ITS RELATION TO SPIRITUAL CHEMISTRY 9 ON THE PHYSICAL THEORY AND PRACTICE OF THE MAGNUM OPUS 38 LIVES OF THE ALCHEMISTS. GEBER 44 RHASIS 46 ALFARABI 48 AVICENNA 51 MORIEN 53 ALBERTUS MAGNUS 57 THOMAS AQUINAS 61 ROGER BACON 63 ALAIN OF LISLE 67 RAYMOND LULLY 68 ARNOLD DE VILLANOVA 88 JEAN DE MEUNG 90 THE MONK FERARIUS 92 POPE JOHN XXII. 93 NICHOLAS FLAMEL 95 PETER BONO 118 JOHANNES DE RUPECISSA 119 BASIL VALENTINE 120 ISAAC OF HOLLAND 123 BERNARD TRÉVISAN 124 JOHN FONTAINE 129