From time to time I make new entries into this continuing series called “Theological Primer.” The idea is to present important theological terms and concepts in around 500 words. Today we look at substance and accidents.


If we want to make sense of the best Christian theologians from the patristic period, through Thomas Aquinas, through the period of Reformed Orthodoxy, and into the 20th century, sooner or later we will need to understand the Aristotelian distinction between substance and accidents.

According to Aristotle’s logic, there is a basic distinction between the thing itself (substance) and what may be said incidentally about the thing (accidents). Substance is whatever exists in and of itself, whereas accidents are what modify substances. Accidents—which might better be called incidentals, so as not to be confused nowadays with our “oops” kind of accidents—bring change to the substance, without changing the kind of thing the substance is. So a dog is a substance, but brown or fluffy or big are accidents. A dog is a dog no matter its color or size, but for brown or fluffy or big to exist, they must adhere to something else. In simplest terms, we can think of accidents as giving to a substance its quality or quantity.

The distinction is an important part of the Roman Catholic doctrine of transubstantiation. Catholic theologians relied on the familiar terms to explain how the bread and wine could still taste, touch, and smell like real bread and wine (accidents), even as the substance had been changed into the physical body and blood of Christ. Reformed theologians rejected the doctrine not only on the grounds that it is historically novel and exegetically fanciful, but also because it is logically untenable. For example, Bavinck, without rejecting the longstanding distinction, argued that in transubstantiation the accidents retain such bread and wine properties that (irrationally) they act more like substances (Reformed Dogmatics, 4.571).

More to the point, most Reformed theologians have readily embraced the terms substance and accidents, seeing the distinction as integral to a proper understanding of God himself. As a simple being, God is not a compound or made up of parts. God is whatever he has. He can be described using terms of quality or quantity, because that is how finite beings can comprehend an infinite God. But as pure being, God cannot be composed of substance and accidents, of matter and form, of potentiality and actuality, or of essence and existence (Reformed Dogmatics, 2.176).

As is often the case, Turretin parses the matter with precision, arguing, “No accident can be granted in God” because of divine simplicity (accidents imply that God is composed of parts), infinity (accidents would add to the substance some new quality), and immutability (accidents always allow for change). God is the Great I AM, the one who is that he is, the one whose essence and existence cannot be augmented by any further properties (Elenctic Theology, IV.1.4).

In other words, if God is whatever God has—that is, if every attribute of God is identical with his essence—then it does not make sense to say that God can undergo a change of any kind (atemporal or temporal, essential or nonessential). God possesses all of his attributes to the uttermost, in such a way that they can be identified with God himself. Consequently, God is only substance, because divinity, by definition, is that true being that can admit no accidents.