Notes on Locke:

Substance/property vs.

reality/appearance vs.

primary/secondary qualities.

© Rick Grush, 2000

There are three distinctions one can find in Locke's work, and keeping them distinct, even when Locke himself does not, is key to understanding his metaphysical and epistemological views. I will here run through the three distinctions quickly, in order to provide an initial idea of the differences between them. I will then go through them again more slowly, pointing to specific passages in the Essay where they are discussed. Finally, I will say something about why they are distinct, and reasons that the are commonly confused. [My notes and thoughts on this are partially inspired by Jonathan Bennett's treatment of these topics in Chapters 3 and 4 of his Locke, Berkeley and Hume: Central Themes, 1971, OUP. As will be apparent, I do not follow Bennett in all respects.]


1. Quick sketch of the three distinctions.

The first distinction is between substance and property. Substance is whatever has, or supports properties (aka qualities aka powers). It is that thing which remains, supplying numerical identity, while properties come and go. The following points are worth keeping in mind.

First, the substance/property distinction applies both in the physical/material and the mental realms, at least for Descartes. That is, for Descartes anyway, material substance supports properties such as this or that shape, size, color, smell, etc. Material substance is the thing that remains the same while a material object changes all such properties. Mental substance supports properties such as this or that idea, this or that desire or intention. Mental substance (a mind) is that which remains the same as different ideas, intentions, etc., come and go.

Second, note that on this (Descartes') analysis of substance and property, properties of extension are also properties. That is, as far as the substance/property distinction goes, colors, sounds, lengths, shapes, etc. are all properties. The same substance may change its properties of extension as well as it can change its properties of color and smell.

The second distinction is between appearance and reality. Locke held a representational view of mind, whereby all that the mind has access to are its ideas, or representations, and the reality that those representations purport to represent is something to which the mind has no direct access. This is often called the 'veil of appearance' doctrine, meaning that all of our knowledge is confined to whatever appears on the mind side of this veil, and the 'reality' behind this veil is necessarily inaccessible to us.


The third distinction is between primary and secondary qualities. These are both properties of matter, and as such are both on the property side of the substance/property distinction, as understood above (but for complicaitons, see Section 2 below). Roughly, the primary qualities are extension, solidity, shape, and such. Secondary qualities are powers that something has to cause certain kinds of ideas in us.

It will be easier to see what this means by first looking at a third sort of property that Locke also recognizes, so called tertiary properties. These are properties that something has completely in virtue of its ability to effect some other thing's primary qualities (and strictly speaking, some other thing's secondary qualities, which it effects by effecting its primary qualities -- see section 4 below). For example, my car key has certain primary qualities -- is a certain size and shape and solidity and hardness. But it also has the ability to effect another object, my car, in a certain way -- specifically, it can open the door lock. Its ability to do this is a sort of relational property between the key and the door lock, in that if you changed either the key or the lock, then the key would no longer have that power.

Now secondary properties are just like tertiary properties, except i) we (humans with minds, and animals with minds too) are the objects that are effected, not just any old physical object; and ii) the effects are limited to causing certain ideas in us. That is, Locke holds that objects cause various ideas in us, like colors and sounds, because they act on our sense organs in ways that are determined by their primary qualities. This ability to cause an idea in us is a secondary quality of the object. It depends on its primary qualities, in just the way that a key's ability to open a lock depends on its primary qualities.


2. Substance and property.

Locke took on board, from Descartes as well as earlier sources, a distinction between a substance and properties of that substance (Locke's focused discussion is in Book II, Chapter 23 of the Essay). In Descartes' famous wax example, the wax changes all its (sensible) properties, all those properties by which we have perceptual access to it, when it is brought near the fire -- its shape, size, color, smell, hardness, etc., all change. Nevertheless, we know it is the same thing, the same wax. How? Descartes' answer was that though we have sensory access only to the wax's properties, we have, through the intellect, access to, or knowledge of, the substance that supports those properties. The properties can change while the substance remains the same. (For our purposes, we can treat the terms property, attribute, mode and modification as more or less synonymous.)

For Descartes, there are two kinds of substance, material (or extended) substance, and mental (or thinking) substance. To an extent, these are treated in parallel. Each is a substance that can support a range of modes or properties, and the properties it exhibits can change over time without endangering the identity of the substance itself. Each, however, has a primary attribute that so to speak defines the kind of substance it is. For material substance this primary attribute is extension, for mental substance it is thought. The different properties that material substance can have or lose are, in the first instance, particular modes of extension. That is, while any material thing must be extended (must have a size and shape), it can change size and shape while remaining the same thing. Minds must think, meaning that there must be some thoughts or other occurring at all times, but there is no particular thought or idea that need remain constant.

Locke begins II:23 with a distinction between particular substances, and substance in general.

Locke's account of particular substances is as follows. We notice that certain sets of ideas commonly go together, like redness, a certain sort of shape, a certain sort of taste, commonly go together in apples. As a sort of cognitive convenience, we suppose that there is something that supports these sensible features, and this we call or think of as the apple's substance. Of course, the only access we have to apples is thought their sensible qualities, such as color, taste, shape, etc., and so we have no cognitive access to this substance of the apple. This apple-substance not only is commonly thought to support the apple properties, but is also thought to provide an explanation of why these properties always go together.

Locke, however, maintains that we have literally no idea of such a substance. Our knowledge of apples is exhausted by our access to them through sensible properties, and especially by our access to their 'powers', which we come to know through the sensible features of apples and things that interact with apples. (Powers come in active and passive varieties. Passive powers are a thing's ability to be effected by something else, such as sugar has the 'passive power' of being soluble in water, while sand lacks this 'passive power'. Acid has the 'active power' of being able to dissolve metal, while alcohol lacks this active power.)

Substance in general is a sort of generalization of our idea of particular substances. While a particular substance is something that somehow explains and supports the properties that something has that set it apart as the kind of thing it is -- something that accounts for why apples and oranges are different, the notion of substance in general is supposed to explain and provide support for all properties, including extension. Apples all have common features, and the particular apple-substance is supposed to explain and support those features. But what set of common features do things in general have? It appears that the only candidate is extension, and so an account of substance in general would be an account of something that explains or provides support for extension. This, Locke says, is senseless.

As Locke says (II:23:2):

We will return to this in sections 4 and 6.

Locke does discuss mental, or spiritual substance. This will be whatever it is that explains or provides support for thought or mentality in general. The idea we suppose ourselves to have of a spiritual substance is of a thing capable of thinking. Locke says that just as we have no idea of corporeal substance, we equally have no idea of spiritual substance -- our knowledge being limited to co-occurring sets of qualities and active and passive powers. For our purposes we can ignore Locke's enigmatic remarks about spiritual substance.



3. Appearance and reality.

Locke also shared with Descartes what we can call a representational view of mind. According to this view, all the mind has access to are representations (the official Lockean line is that all ideas are either sensory ideas, or ideas of reflection (introspection on the mind's own operations), whereas Descartes allows other kinds of representational capacity and sources for them).

A feature of representations (ideas) is that they can misrepresent: the fact that one has a representation that purports to exhibit some state of affairs does not at all entail that that state of affairs actually obtains. There are not only perceptual illusions, and hallucinations, but even dreams which seemingly create an entire world or alternate reality that need not in any way reflect what is actually the case. The representational view of mind opens a difficult if not impossible to bridge gap between what one can know (the representational contents or ideas within the mind), and what the world is really like. (Descartes' official line to a first approximation was that God, being perfectly good and hence no deceiver, guaranteed the accuracy of the core of Descartes' representations of the world. It is thus God's benevolence that closes the epistemological gap between mind and world.)

Locke's treatment of the epistemological predicament that his representational theory of mind creates is highly inadequate. There are two related aspects to this inadequacy.

First, Locke stipulates without good argument that our ideas of the primary qualities (extension, etc., see Section 4) of things are accurate, that is, the things without us, the things on the other side of the veil of perception, actually have the primary qualities that we perceive them to have (though, of course, we cannot perceive all the details in something's primary qualities, as there will always be microscopic features such as textures and surface properties that we will not have perceptual access to). As an aside, it was more or less the same features bout which, according to Descartes, God guaranteed the accuracy of our ideas.

Locke held a causal theory of perception, according to which external things, things in reality, cause us to have certain ideas via causal interaction between those entities and our sense organs. While this might be a decent scientific theory of perception, where a scientific theory is one that takes the existence of the real, external world for granted, it is inadequate as a philosophical theory, because the existence of the external world, including the putative causal relations, are all things that lie on the unknowable side of the veil of perception.

Second, Locke's arguments to the effect that we can know that there is an external world behind out perception are bad. There are two sorts of consideration he marshals. First is an appeal to what might be called common sense -- he just claims that we know this, and anyone who doubts it is either crazy or joking or something like that. Second is a simply bad argument to the effect that some of our ideas come to us without our willing them, and that it is obvious that those that come unbidden -- such as the ideas of roundness and brightness when I open my eyes and look upward at noon -- can only be explained in terms of the causal effect of an external object.

Locke's official view of epistemology -- that all the mind has access to are ideas ultimately derived from sense experience -- forces an epistemological view that opens a gap between appearance and reality. Locke wants to, and tried to, close that gap, at least for primary qualities, by stipulating that the ideas of primary qualities are accurate, and by appeal to the success of a certain kind of physiological account of perception by which we perceive things as a result of the causal impingement of those things on our sense organs. But the stipulation by itself is an untenable one within his epistemological system, and the appeal to physiology simply begs the question. All of one's putative knowledge of 'sense organs' and causal interaction between sense organs and perceivers could simply be part of the big dream, the big deception, there being nothing corresponding to such notions in reality.



4. Primary and secondary qualities.

Locke actually draws a three-way distinction between primary, secondary, and tertiary qualities (these last two often called 'powers'). (Locke's discussion here is most focused at Essay 2:9:23). Primary qualities are bulk, figure, number, situation, motion/rest. Roughly, these are the properties that a body has considered on its own -- non-relational properties, as they do not depend on any relations between that body and anything else.

Secondary qualities are powers that a body has to produce ideas, such is of colors, sounds, smells, etc. in us. A body will have these powers in virtue of its primary qualities, and in virtue of the constitution of our own sense organs. Locke thinks that it is the microscopic features of the shape or texture of surfaces that explains why we see them as this color or that, for example. For instance, it is the very small-scale primary qualities of an object (such as the motion of it's small parts), that causes us, because of the constitution of our sense organs (thermoreceptors in the skin) to have the idea of warmth. This secondary quality depends on the objects primary qualities, and also on the primary qualities of our sense organs. It is, then, a sort of relational property between the primary qualities of two different things -- just like the property of being able to open a lock is a relational property between the primary qualities of the key and the primary qualities of the lock.

Tertiary qualities are powers that an object has to effect (and be effected by) the secondary (and primary) qualities of other objects. For example, a burning log has the active power of being able to change the secondary qualities of wax, so as to make the wax appear different -- that is, so as to give the wax different secondary qualities, and hence to cause different ideas in us. Let us take as an example, the fact that the burning log has primary qualities, such as motion in its parts, and it then effects the primary qualities of the wax, in this case by causing faster motion in its parts. This new primary quality of the wax amounts to a new secondary quality, because the wax will now be able to cause in us the idea of warmth, different ideas of colors than it did before, etc.

This tertiary quality of the burning log is an active power. By the same token, the wax has the passive power of being alterable by the log: it is so configured that its primary, and hence secondary, qualities can be altered by the burning log.



5. Why the three distinctions are different distinctions.

In this section I want to show that, in principle anyway, the three distinctions are themselves distinct. Now this does not mean that on certain assumptions one or more of them might not line up, but that as far as the distinctions themselves go, they might not line up at all. In order to show that they are distinct, I will explain three things: (i) how appearance/reality differs from primary/secondary; (ii) how appearance/reality differs from substance/property; and (iii) how primary/secondary differs from substance/property.

(i). It is easy to see why the reality/appearance distinction is not the same as the primary/secondary quality distinction. First, note that only someone who thinks that things really are extended can possibly assume that reality lines up with primary qualities. Kant, for instance, maintained that there was a super-sensible reality that was behind the veil of appearance, but that the things in this reality were not extended. In Kant's case, primary and secondary qualities are both part of the realm of appearance, and neither applies to things as they 'really are'.

On the other hand, if Descartes were in fact just a mind being deceived about the nature of the world by an evil demon, the appearances would be non-extended ideas, and the reality would be a non-extended deceiving mind/spirit. In this case, there is an appearance reality distinction, but neither primary nor secondary qualities would apply to anything on either side.

Also, one might consider that for an omniscient God, there is no appearance/reality distinction: Such a God has direct knowledge of things as they really are. Nevertheless, there would still be a difference between the primary and secondary qualities of objects, even from the point of view of this omniscient God.

In short, the appearance/reality distinction is a distinction between what we do and what we do not have direct mental access to. The primary/secondary quality distinction is between two kinds of properties that things may have, either or both properties might fall one either side of the appearance/reality divide, and there could be such an appearance/reality divide without any extended properties, primary or secondary.

(ii). Also, it should be clear that the appearance/reality distinction is not the same as the substance/property distinction. There might very well be substances AND properties that are all on the 'reality' side of the divide, and aboput which we have no ideas at all. God might have properties that we can never have any inkling of, for example. So one cannot equate reality and substance. Furthermore, one cannot equate property and appearance, since it might be the case that substances are on the 'appearance' side of things. One could interpret Descartes as claiming that one's own mind -- a thinking substance -- is directly perceived. More centrally, for Kant, substance is itself a category that applies only to appearances. Things as they really are are not substances in the same sense, at least not for Kant.

(iii). The primary/secondary quality distinction does not necessarily line up with the substance/property distinction. First note that even for Locke, primary and secondary qualities are both properties of bodies. Neither is 'substance in general'. (Though there is some reason for thinking that primary qualities might be a sort of 'substance' of particular substances -- see sections 4 and 6.) A substance in general would be something that would have, or support, all properties, including properties of extension, shape, size, etc.



6. How the three distinctions can (and in Locke for the most part do) get lined up.

Locke's doctrines entail a certain amount of overlap between the three distinctions. The overlap is created by the assumption that real things, the things one the reality side of the appearance/reality distinction, are extended, and thus really have primary (and secondary, and tertiary) qualities. Now this overlap is not forced by the distinctions themselves, as we saw in section 5. Rather, it follows from a claim about the nature of reality. This at least explains why one might identify reality with primary qualities. But it does NOT allow one to identify appearance with secondary qualities. Secondary qualities are, on Locke's view anyway, as much a property of real things as they really are as are primary qualities -- in fact the primary qualities of things give them the secondary (and tertiary) qualities they have. (Note that secondary properties are relational properties. They are properties of the objects nevertheless. Given the constitution of the door lock, it is a porperty of the key that it can open the lock -- it is not just an appearance that it can do this.) The confusion is caused because on Locke's view, the secondary qualities cause ideas in us, and ideas are on the appearance side of the appearance/reality distinction. But one needs to realize that secondary qualities and the ideas they cause are two different things, just like the sun and the sunburn it causes are two different things. So the fact that the ideas are on the appearance side does not entail that the secondary qualities are on the appearance side. (Note that if any confusion of qualities of objects and appearance is licensed by Locke's position, it is a confusion between PRIMARY qualities and appearance, because Locke claims that our ideas of primary qualities -- our ideas of extension, etc. -- actually resemble things as they are, while our 'ideas of' secondary qualities do not. In other words, for Locke, secondary qualities are not as they appear (redness of our idea of the apple is not actually in the apple), while the primary qualities are. But strictly speaking, even this confusion is not allowed on Locke's view.)

But how does the substance/property distinction fit into the appearance/reality and primary/secondary distinctions for Locke? This is where things get really difficult, but also interesting.

It depends on whether one is talking about 'substance in general', or 'particular substances'.

If one is talking about 'substance in general', then Locke seems to waffle between two positions. The first is that there really is no such thing, it is a meaningless abstraction from the useful notion of particular substances. On this interpretation of Locke, then 'substance in general' is neither appearance nor reality, and it would also be neither a primary nor secondary quality (thought it is, on this reading of Locke, the thing that people confusedly think underlies all properties, including extension, etc.). The second position is that substance in general is real, a part of reality, but it is something that we can in principle have no knowledge of. At best, we can have knowledge of its properties, including its properties of extension. On either reading, substance in general is not the same thing as primary qualities. One the second reading, but not the first, it is part of reality, but a part we can have no idea of. On the first reading it is not even a part of reality, it is just a confused posit of the mind constructed by illegitimately abstracting from the notion of particular substances.

But what about particular substances? Locke is not entirely clear about this, but I think one can interpret him as saying that the primary qualities of things are something like the 'substance' of particular substances. (It should be noted that this interpretation coul dnot be agreed upon by all Locke commentators.) I will start explaining this in the next paragraph. But note that if this interpretation of Locke's view of particular substances is right, then primary qualities, the 'substance' of particular substances, and reality all line up in that a particular thing's primary qualities are part of reality, not (just) appearance, and are also in a sense its substance, in that the primary qualities support and explain the properties and powers that a particular substance has. (Keep in mind that even on this view, the secondary qualities are also part of reality, and not appearance.)

Now, why would one think that for Locke a thing's primary qualities are the 'substance' of particular substances? Note that substance is supposed to do two things: it is supposed to explain how something can remain the same thing while it changes properties, and it is supposed to be that unknowable thing in which its properties 'inhere'. According to Locke's treatments of what makes particular substances the things they are, and his account of secondary and tertiary qualities, it turns out that primary qualities do a good job of filling the bill of being substance.

On Locke's account, the best idea we can have of a substance is the collection of simple ideas it causes and its active and passive powers with respect to other particular substances. We have no idea of the 'substance' that underlies or explains these. But note also that on Locke's view it is a thing's primary qualities that underlie and explain these secondary and tertiary powers. So how can Locke say both of these things? Well, Locke claims that although it is a body's primary qualities that support its secondary and tertiary powers, we have no idea of these for the simple reason that they -- the micro-textures, motions of minute parts, etc. -- are too small to be seen by us. So according to Locke we really do not know the substance underlying the redness of an apple. We know the sort of thing it is: some sort of arrangement of primary micro-features. But we have no idea of the details of what these are and how they work.

The difference between this notion of substance -- as the unknown primary micro-features that account for the particular collection of powers that a particular substance has -- and the much-abused-by-Locke notion of 'pure substance in general' seems to be that this latter notion would have to be something that underlies even primary qualities.


© Rick Grush, 2000