5.2 The Nature of Reality: Substance or Substances


By the end of this section you will discover:

  • What philosophers have meant by the term “substance.”
  • The meaning of “ontology.”
  • How Aristotle made a distinction between the substance of a thing and its “attributes.”
  • How Descartes, Spinoza and Locke understood the term “substance.”
  • What the possibilities are for assessing the substance(s) of the universe.

Philosophers have employed many different terms to describe the fundamental “stuff” of which reality might be composed. A key question has long been whether reality is composed of one kind of stuff or of multiple kinds of stuff. The classical term for the stuff of reality is Substance.

What is a Substance?

One of the most important ontological questions (i.e., questions about what things can, do, or must exist, and how they should be categorized) is about what the fundamental constituents of reality are. We all know that there are tables and chairs and people and planets and cows and trees and so on. But what are these various things – what is reality – fundamentally “made up of”? Is reality just atoms and void, as the pre-Socratic Democritus famously claimed? Or is the fundamental stuff of existence water, as ancient Thales seemed to have believed? Was the pre-Socratic Anaximander correct everything emerges from air? These are all attempts at seeing physical matter as the basis of existence,  But what of our minds and their ideas, are these not also fundamental parts of reality? Or indeed, is the non-physical mind the very “stuff” of reality?

These can all be understood as questions about substance. Unfortunately, this is one of the most confusing terms in all of philosophy, largely because different thinkers throughout the ages have used it in so many different ways. One of the most influential accounts of substance was presented by the ancient Greek philosopher Aristotle (384-322 BCE).

As we read in section 4.2.3 on Empiricism, Aristotle believed that reality lay in particular, individual things.  This horse, here, is real.  The category “horse” is an abstraction.  The being (Gr. ousia) lies in the particular, existing horse, not in categories like species or genera or familia that can be added to it.  Thus, this particualr horse is a substantial thing, but the categories to which this particular horse belongs are “accidental,” not essential to the existence of the horse (see below).

Two horses standing in the grass in a field
Wikipedia “Horse”






Scientific classification

Domain: Eukaryota

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Perissodactyla

Family: Equidae

Genus: Equus

Species: E. ferus

Subspecies: E. f. caballus



As Marc Cohen explains this:


“The idea here seems to be that what makes species and genera secondary is that they are just kinds or collections. A species is just a collection of individuals, and a genus is just a wider collection of the individual members of the species that fall under it. Without those individuals, there would be no species, and without the species there would be no genera. For the species tiger to exist, for example, is just for there to be individual tigers. It is the individual tigers and the other individual plants and animals that are the real things; their species and genera are simply the way the specimens are classified and organized. The species and genera of non-substance categories, such as red and color in the category of quality, are doubly dependent. For they are collections of individual qualities which are themselves ontologically dependent on substances.” (S. Marc Cohen, “Substances,” https://faculty.washington.edu/smcohen/Substances.pdf, accessed 11/12/2023)


As Aristotle explains it in his Categories:


A substance—that which is called a substance most strictly, primarily, and most of all—is that which is neither said of a subject nor in a subject, e.g. the individual man or the individual horse. The species in which the things primarily called substances are, are called secondary substances, as also are the genera of these species. For example, the individual man belongs in a species, man, and animal is a genus of the species; so these—both man and animal—are called secondary substances.[6]
Another way Aristotle tries to explain substance is by seeing it as something which is essential, unchanging in a particular thing (this horse) while all else that can be said of it (brown, tall, fourfooted, etc.) are “attributes,” qualities that are not unique to the particular object (e.g.: other things can be brown, tall, four-footed, etc. but only “horse” can be said of horse).  Such a substance–attribute theory tells us that objects are constituted by a substance and attributes associated with the substance but which are distinct from it.   What Aristotle tries to get us to understand is that some things we can say about a thing are essential, they make that thing unique from all other existing things, while other things are non-essential (or, “accidental”), designating qualities a thing shares with other existing things.


This is close to what Descartes meant by substance.  For Descartes a substance is a reality which exists in such a way that it needs no other entity in order to exist. Therefore, only God is a substance in this strict sense. However, he extends the term to created things, which need only the concurrence of God to exist. He maintained that two of these are mind and body, each being distinct from the other in their attributes and therefore in their essence, and neither needing the other in order to exist.  These are the two substances which make up Descartes’ mind/body dualism (more later in this chapter).


For Spinoza, on the other hand, there could be only one substance, which he called God.  This one substance can be conceived of as material and also, at the same time, as mental. What is ordinarily called the natural world, together with all the individuals in it, is immanent in God: hence his famous phrase deus sive natura (“God or Nature”). Because he denied Descartes contention that the universe is made of two substances, Spinoza is considered a metaphysical monist.
John Locke, indebted to Isaac Newton, understood that the “Primary Qualities” of an object did exist independently of minds, and thus reiterated a form of substance dualism.  There is a physical world distinct from the mental.  Locke’s empiricism took him away from metaphysical abstractions about substances and essences so that for him objects simply are what they are – made up of microscopic particles existing because they exist. According to Locke, the mind cannot completely grasp the idea of a substance as it “always falls beyond knowledge”. (Dunn, John (2003). Locke: A Very Short Introduction. Oxford University Press.)


From all of this we can see that the notion of substance is problematic and has been used differently by different philosophers.  For our purposes we shall understand it to mean what a thing is in its essence, what it still is when all unnecessary characteristics we might say of it have been stripped away.

What are the Metaphysical Possibilities?

In the next few sections, we will explore some options for the composition of reality. This chart sets forth the main possibilities.





the universe is composed only of physical matter


the universe is composed only of non-physical thought


the universe is composed of two substances, Matter and Thought that interact with each other.


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PPSC PHI 1011: The Philosopher's Quest by Daniel G. Shaw, Ph.D. is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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