This second volume of Hervaeus Natalis's polemical work, De quattuor materiis contains his De esse et essentia. In this work the author criticizes the rival systems of the metaphysics of creation that were upheld by Giles of Rome and Henry of Ghent, and presents an exposition of his own notion of being. To explain Harvey's antagonistic attitude to Henry of Ghent and his simultaneous rejection of Giles's positions (the rigid Aegidian real distinction between essence and existence in particular) it was necessary to provide a thorough investigation of the ontological positions of both Henry and Giles. Hence the lion's part of the Introduction is devoted to these two rivals of Harvey's.The selection of the manuscripts used for the present edition of De esse et essentia as well as the ratio edendi, orthography, punctuation and headings employed, are explained in the General Introduction to volume one, De formis (SA 30).This second volume had been finished by the editor, L.M. de Rijk, just before his sudden death on July 30, 2012. The final version has been read by Joke Spruyt and Olga Weijers.The third and last volume of the edition of Hervaeus' work, already well advanced by the editor, will be finished by two of his main disciples: Henk Braakhuis and Onno Kneepkens. Thus we will have kept our promise, in respect and friendship for our master.