This book looks at the medieval text, On Being and Essence (De ente et essentia), a strictly metaphysical work by Thomas Aquinas. The author thus considers terms such as matter, essence, accident, actualization, real composition, individuation – expounding on how this terminology is important for understanding the work of this famous medieval philosopher. In this interpretation, the author draws on existential Thomism, a neothomist school of thought that is directed foremost on the relation of essence and being in Aquinas’ oeuvre.