Thoughts on Aquinas's Two Conceptions of Essence

Trying to grapple with the two conceptions of essence in Aquinas's thought. I might be totally wrong about my conclusions... If anyone knows better, please correct me.

Essence: the constitutive principles or nature of a thing (substance), distinguishing it from other substances, and from which flows its proper and accidental operations.

According to Thomas Gilby, O.P., [1] nature in St. Thomas Aquinas's thought can be analyzed in horizontal or vertical terms. A horizontal conception of nature is Roman/Latin, structured, disciplined, categorizing substances in similar planes, according to genus and species. An example of this analysis in the concrete is the Roman Colosseum in which a simple idea (the Etruscan/Roman arch) is repeated over and over again to form a structure. The supernatural in this conception is relative to what horizontal plane one is analyzing. Here, vegetable, sensory, rational, divine are superimposed. The supernatural is relative based on what is proper to being that occupies higher or lower planes. Form from above is imparted via participation; per participationem rather than per essentiam.

A vertical conception of nature is more Greek than Roman, biological than typological, and analyzes substances with respect to inner dynamics (morphe) rather than its status at any particular moment in time (eidos/horizontal). The definition of a being is not so much its fixed place in terms of genus and species but its telos. Such a vertical analysis looks at the constitutive, sublating properties in successively-higher structures or forms of being; e.g. inanimate, vegetative, locomotive/sensory, rational. In this analysis, dynamic structures are constituted by smaller, "lower" parts or properties (causes) but irreducible to them, such as a car and its individual components, or a symphonic piece.

These two conceptions are not incommensurable in the Kuhnian sense (mutatis mutandis) but mutually- and simultaneously-inter-formative. They provide the difference between necessary contingent causality and accidental contingent causality.

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Notes:

1. Thomas Gilby, “Appendix 8: Natural and Supernatural,” in Summa Theologiæ: Volume 1: Christian Theology (1a.1), trans. by Thomas Gilby (Cambridge: Blackfriars, 1964), 99-101.