title: On Being and Essence (Mediaeval Sources in Translation)
author: Thomas Aquinas
description: Because a small error in the beginning grows enormous at the end, as the Philosopher remarks in Book 1 of On the Heavens and the World, and being and essence are the first things to be conceived by our understanding, as Avicenna declares in Book 1 of his Metaphysics, in order to avoid falling into error about them, and to reveal their difficulties, we should see what are signified by the names of being and essence, how these are found in various things, and how they are related to the logical intentions of genus, species, and difference. And since we need to arrive at the cognition of simple components from the cognition of what they compose, and from those that are posterior to those that are prior, so that the discussion may suitably progress from the easier subjects, we should proceed from the signification of the name of being to the signification of the name of essence.
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  Existence and Subsistence in St. Thomas 
   
   
   
   
  Jeffrey C. Kalb, Jr. 
   
  This text is hereby released into the public domain. 
   
   
   
   
  Beatae Mariae Semper Virgini, 
  Mediatrici, Coredemptrici, et Advocatae
  
   
   
   
   
  
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  I. Existential Thomism 
   
  For over fifty years, the dominant interpretation of the metaphysics of St. Thomas Aquinas in 
  academies and philosophical publications has been that of existential Thomism. Its influence is 
  marked even among those Thomists who eschew the label. This interpretation endows to-be 
  (esse), that is, the act-of-being (actus essendi), with a purely existential character, making it in 
  every respect ontologically prior to essence (essentia). Out of this extreme position are drawn the 
  existential Thomist’s peculiar polemics against “essentialism.” For the purposes of this essay, 
  and in agreement with common usage, an “essentialist” metaphysics will be defined as one that 
  accords some priority to essence (essentia) over to-be (e