The question which seeks the essence of something, (ti ēn einai), according to the argument of this thesis, was fashioned by Aristotle because of ambiguity or 'homonymy' inherent in the nature of universal predicates. However successful the conceptual analysis of universals may be as such, their meaning or significance cannot be fully fixed or determined except as a function of the subjects to which they are applied. The distinction between understanding a universal predicate as such and understanding its application to a particular subject may be roughly expressed as that between the ability to recognize the presence of an attribute in a subject and the knowledge of what the predicate says about the subject. It is in order to transform knowledge of the first kind into knowledge of the second that the 'essence-question' is asked. It is shown that the Aristotelian notion of an essence (to ti ēn einai) is explained through the notions of a subject (ypokeimenon) and of an actuality (energeia). Aristotelian 'essences' express the actuality or activity of a substance conceived from the 'categorical' point of view as the subject of qualities and universal predicates in general. An 'essence', insofar as the term applies to sensible substances, is the being of something as the subject of qualities and material predicates, i.e. universal predicates in general. Entailed is the denial that an essence in Aristotle's sense is constituted by attributes, characteristics, or universal predicates of any sort whatsoever. The argument exploits the distinction drawn by Aristotle on a number of occasions in the Metaphysics between material substrata of a substance and the subjects of qualities. The development of the position hinges on an analysis of matter and form in terms of the relations of potentiality and actuality conceived as contemporaneous modes of existence.