The aim of the present edition of Harvey Nedellec's De quattuor materiis is to make a collection of texts available that can throw some more light upon the ongoing debates around 1300 about some highly controversial issues, including the plurality of forms, the relationship between being and essence, the significance (or superfluity) of the intelligible species, and the intellect's priority to the will. Harvey's polemic interventions, which are explicitly directed against the ontological positions held by Henry of Ghent, are the more interesting as they are coloured by a manifest animosity against his opponent and the Ghentian way of doing philosophy in general. The author's attitude is most prominent in the first tract of the collection presented in the first volume, De formis. In order to put the impact of this tract into a larger perspective, Harvey's extensive treatise De unitate formae substantialis in eodem supposito has been added.