Issue #22 April 2019

The Irreducible Instant: Descartes’ Thinking Substance

László Moholy-Nagy, “A IX”, (1923), (Detail)

hat Descartes “thinks therefore he is” is well known. On the hunt for a fragment of absolutely necessary and indubitable knowledge, Descartes hits upon a class of existents that, even if we were wrong about absolutely everything, we could still maintain an iron-clad confidence in. This class of existents is mostly attributed to the Cartesian “I” as their ‘bearer’, but at other points in the meditations they are identified with this “I” as its constitutive elements (“But how often [do I exist]? Just when I think.”(AT VII. 27)).

Here I want to explore Descartes’ thinking substance, having already examined his ‘extended substance’ (which turned out to be a harmonically vibrating void-stuff). What is thinking, according to Descartes? And why was he so sure that it formed its own unique and irreducible substance (i.e., why couldn’t it be reduced to a general materialist picture of brain functioning)?

Following Descartes through, we end up seeing a strong connection (noted by Kant) between thought and time. Descartes seems to have missed this (believing time to be derived from motion, itself derived from extension). But, by pulling on the threads here and there, we find in Descartes own writing the beginnings of a solution to the problems of dualism.

But, first, what is thinking?

Definition From ‘The Meditations’

In the Objections and Replies in the Meditations, Descartes offers a ‘geometric proof’ of the book’s arguments. This would obviously be a good place to find Descartes’ attempt at the most rigorous definition of what he took the thinking substance to consist in:

“(Definitions 1.) Thought is a word that covers everything that exists in us in such a way that we are immediately conscious of it. Thus all the operations of will, intellect, imagination, and of the senses are thoughts. But I have added immediately, for the purposes of excluding that which is a consequence of our thought; for example, voluntary movement, which, though indeed depending on thought as on a causal principle, is yet itself not thought.” (AT VII. 160)

This immediacy, though Descartes mentions adding it for the purposes of separating thought as cause from its effects, is needed regardless: this thinking substance, whose modes he has listed, is discovered through the process of radical doubt (a subtractive method). Its nature, then, needs to have this immediacy because any non-immediate (mediated), seemingly mental mode can be doubted. For example, when I perceive a chair here it very quickly suggests to me the belief that there is a ‘chair-like’ body out there in the world, beyond my perception of it, causing this perception. But this ‘very quickly’ cannot dodge the regime of doubt: my inference may be flawed, there may be no chair but just the trickery of an evil demon. However, it seems indubitable that I am now having a perception of a chair-like thing, regardless of whether the reason (ultimate cause) for this perception is a powerful demon, or a benign chair-body in the world, or a complex computer program, or anything else. The perception, considered in itself, has an immediacy that an inference or deduction based on it, no matter how ‘intuitive and swift’, does not.

Likewise with the intellect being the cause of the effect of my belief in a certain calculation, say, 5+7=12. Descartes admits that insofar as the calculation is not intuited immediately, only very quickly, it is possible for an evil demon to substitute a faulty answer every time I perform the calculation (AT VII.21). That I am calculating is immediate, as is as my thinking now that the result is 12, but the product of this calculation falling naturally and necessarily out of its terms can be doubted.

This immediacy is so essential that Descartes does not begin with an idea of thinking as necessary (the indubitability of doubt), and then go on to compare this idea to other things (perceiving, willing, etc), discovering at the end that they too all possess this immediacy, rather he works in the opposite direction: all that possesses this immediacy, insofar as it possesses it, is thought. This is why perception gets added to ‘thought’ because it shows this same immediate (and thus indubitable) character when taken directly (as just being what it is, and not considering what it purports to signify or indicate).

The Immediate

What is this ‘immediacy’ that is the defining attribute of thought? Immediacy stands to thought, which stands to ‘thinking substance’, the way that ‘shape’ stands to extension, which stands to ‘extended substance’. That is to say, of the two substances they have two principle attributes: extension and thought. A necessary consequence of these states of affairs is that there are shapes and ‘immediacies’ because that’s what being extended and being thought mean, respectively.

Unfortunately, we can only indicate emphatically towards this ‘immediacy’, not determine it: any conceptual determination would be a potentially true or potentially false ascription of a mode to this immediacy, not the immediacy itself. Unlike in the case of extended substance where we permitted ourselves the assumption that external bodies existed (thus allowed ourselves the conceptual determination of ‘body’ to work with), the divination of the essence of mind gets the entire project moving in the first place by making no such assumptions. So, we are left with an immediacy that we can only indicate to.

This is why I disagree with those commentators who would see the translation of the 1st personal character of Descartes arguments (I, We) to a general 3rd person (minds in general) as a non-controversial move (for example, Skirry, 2018). Descartes has not proven that minds per se exist necessarily, only that our minds exist necessarily (to be fair, I’m not sure what the import of this distinction is, but feel the move from the 1st to 3rd person needs some additional argument to support it). This means that Descartes can, through his argument for the necessary existence of thought, only indicate to each of us this immediacy that inheres in thought, but cannot do this through a 3rd person demonstration or proof because this immediacy can only be indicated, not determined. This is why in all of his succinct definitions of it he contradicts himself: using words like “in”, “inner”, or phrases like “we are conscious of” (the consciousness that we are conscious of?). By Descartes own arguments these spatial terms he keeps falling back on for indicating to the thinking substance are illegitimate: only the extended substance allows for the use of spatial concepts. The immediacy that marks the thinking substance can only be indirectly indicated, or demonstrated through the Cogito ‘procedure’. A succinct definition can only contradict itself in trying to capture it (because of the mediated subject-predicate structure of elements of a proof). However, each of us on our own ‘gets’ it when it is indicated to.


Another way of getting at this immediacy is to consider how early on in the second meditation, before having his eureka moment, Descartes presents a list of things he believes now may be called into doubt (based on the ‘evil demon’ presupposition):

“I suppose, then, that all the things that I see are false; I persuade myself that nothing has ever existed of all that my fallacious memory represents to me. I consider that I possess no senses; I imagine that body, figure [shape], extension, movement and place are but the fictions of my mind.” (AT VII. 25)

Note that Descartes does not refer to particular bodies, shapes, or movements, but the general notions. That is, in his radical doubt, Descartes sees it as a conceivable possibility that there is, in fact, no space or shape to be had in the universe. Movement and memory are likewise questioned here. The former seemingly because it derives from extension, but in fact needn’t, because a moving illusion (for example the ‘rotating snakes’ illusion) still presents motion as a perceptual category: nothing needs to be ‘actually’ moving for a concept of apparent movement to maintain itself. So, when Descartes, in his scepticism, entertains the possibility that movement is an illusion, he is entertaining that even ‘illusory’ movement is an illusion!

This makes more sense when we consider that memory is added to this mix of ‘doubtables’: what Descartes is thus picturing to himself here is a mind that has been cast into a newly fabricated universe at this precise moment, along with its (false) memories of previous moments, as well as its (false) memories of the objects it now believes itself to be watching occupying slightly different positions a fraction of a moment ago (there is no movement, even among perceptions). For us to follow Descartes into a doubt that would, on the one hand, affirm the indubitable character of present, immediate, perceptual experience, yet on the other, deny (in theory) movement and memory, is to posit a universe that exists perfectly as single ‘frame’ (in the sense of a frame of film) of consciousness with no past and no future. This frame right here, occupying a slither of the absolute minimum of continuous thinkable time.

Now, it’s an extreme, though tenable, form of scepticism to posit that the entire universe was created two minutes ago, and it’s no more extreme (and just as tenable) to posit the universe (whatever it is composed of) was created zero time units before this instant right now (i.e, is created just now at this instant), and exists only for the absolute minimum of continuous thinkable time before disappearing. Pop! But even this near psychosis level of scepticism that believes in only a single freeze-frame still cannot overcome the limit of the Cogito: it is a step too far to posit that the universe is to be created in some minimum of thinkable time from now. That is, it is yet to be and for now there is nothing. This is contradicted by the uttering of the very claim, akin to doubting that one doubts when they doubt. The Cogito works completely on this infinitely thin knife edge of the present instant, and, in fact, is no ‘thicker’ than this at all. It’s this immediacy that lies at its core, both in the sense of being im-mediate, as unmediated, and also in the temporal sense of occupying no time at all.

But all of this madness (the freeze-frame universe) undermines Descartes own list from above, not by being inconceivable (because it’s not), but when we whip ourselves into conceiving it, we realize there is a stray indubitable in there: extension. Even if we can somehow conceive of the very limits and essence of the immediacy of thinking by trying to intuit that the evil demon has created merely one, immobile ‘slice’ or frame of consciousness with no temporal extension, when we examine this slice, we see it is still wound through with spatial extension. When Descartes says he supposes there to be no motion, nor memory, and no extension or shape, he’s gone too far. Even if we can fully doubt the positive existence of the past, and the becoming of the future, our direct phenomenal experience right now still presents us with extension and shape. Immobile extension and shape, sure, and the extension and shape of nothing but an appearance in a frame, but even the sense of touch delivers us a certain simple perpendicular arrangement, an experience of a line or point, even if this instant of touching this table with my eyes closed is all that there ever was or will be. The immediacy of the thinking substance seems intimately linked to the shapes and figures of the (supposedly separate) extended substance. What’s going on here?

At this point it will be instructive to see how Descartes modified his notion of ‘thought’ in the Principles.

Wassily Kandinski, “Free Curve to the Point — Accompanying Sound of Geometric Curves“, (1925).

Definition from ‘The Principles of Philosophy’

Descartes, in Part I, Art. 9 of the Principles, first offers the same definition of ‘thinking’ as that in the Meditations. He then goes on to make further distinction in Part 1, Art. 32, between “the perception of the understanding” and “the action of the will”. He refers to these two as “general classes” of thinking. However, he then performs an interesting back-track:

“There are, however, besides these [thinking and extended substances], certain things of which we have an internal* experience that ought not to be referred either to the mind of itself, or to the body alone, but to the close and intimate union between them, as will hereafter be shown in its place. Of this class are the appetites of hunger and thirst, etc., and also the emotions or passions of the mind which are not exclusively mental affections, as the emotions of anger, joy, sadness, love, etc.; and, finally, all the sensations, as of pain, titillation, light and colours, sounds, smells, tastes, heat, hardness, and the other tactile qualities. (Principles, Part I, art. 48) — (* Again, this ‘internal’, to stay true to Descartes own premises, has to be taken as a figure of speech)”

What makes this passage highly interesting is his sudden retraction of sensations (perception, lights and colours, sounds, smells, etc) from the substance of thinking simpliciter. Hitherto, the thinking substance has had perception/sensation as one of its modes (owing to their indubitable immediacy), but here Descartes adds an extra layer of complexity: perception simpliciter does not belong purely to the thinking substance, and its immediacy, as he has argued up to now, but are referred to the union of thinking and extended substances. Thus, perceptions are a kind of extended thinking, or immediate extension.

That is, even in the extreme skepticism at the knife-edge of the thinking substance, the freeze-frame universe, this freeze-frame is organized down spatial lines. Taking a region of the frame, this present perception of this green leaf for example (a ‘qualia’ in the modern parlance), we can’t chase the immediacy of the thinking substance all the way through to conceivably deny its shape, nor can we simply attribute this green shape to an extended corporeal substance completely.

We cannot attribute it to extension, because in Descartes’ view of extended substance, there is only ‘mobile extension’, not ‘coloured extension’. Descartes argues that motion alone can only account for the cause of the experience of colour as a series of motions within the nervous system and brain (Principles, Part IV, Art.198). From the 3rd person point of view of extension, we only find movement, a central nervous system and optic nerve moving in complex ways in regards to the oscillation of a light wave (itself a movement).

However, seen purely from the perspective of mind, we can no better understand this colour of the leaf, for we cannot ‘imagine’ a pure colour without shape, that is, without being determined by the attribute of extension. So in the direct experience of some coloured qualia, even if merely imagined, there exists something both immediate and extended, with neither of these attributes being extricable without abolishing the ‘phenomenal body/qualia’ we are imagining/perceiving.

However, now, if my way of presenting this picture is accurate, we can look both to extended substance and thinking substance in isolation and attempt to distribute what belongs solely to each, the one and the other, and not their union. Doing this, we will see that there is only two slithers of area without overlap. Pure geometrical extension on one side, pure immediate thought without ‘locality’ on the other. The pure extended and thinking substances pale in comparison to their union to such an extent that we should reconsider what we have been terming ‘the world and the mind’ all along. The given of experience is always immediately extended.

Vera Molnar, “Structure de Quadrilateres (Square Structures)”, (1987)

The thinking substance possesses an immediacy that forecloses any possibility of conceiving that the present instant is not what it is, and this immediacy is then tied up with perceptions that are everywhere and always disclosed as being organized into ‘regions’, thus foreclosing any possibility that extension, of a general sort, could be doubted completely, even if we admit the possibility that extended non-immediate (unthinking) matter might be a fiction.

Furthermore, when attempting to illustrate the nature of extended substance in the Principles Descartes avails himself a number of times to an explanatory resource: where the rational implications of a pure extended substance seem to conflict with our most basic experiences, he appeals to our experiences being motivated in such and such a way. So, for example, when we speak of the empty vacuum of space, what we really mean is not that it is empty (for it is filled with extended substance) but only that we find nothing of interest there. ‘Absence’ is thus a mode of ‘attitudinal extension’: I open a box and say it’s empty because it does not contain anything I could desire or manipulate.

“Space or internal place, and the corporeal substance which is comprised in it, are not different in reality, but merely in the mode in which they are wont to be conceived by us. (Principles, Part II, Art. 10)

And, in truth, by the term vacuum in its common use, we do not mean a place or space in which there is absolutely nothing, but only a place in which there is none of those things we presume ought to be there. (Principles, Part II, Art. 17)

the perceptions of the senses are merely to be referred to this intimate union of the human body and mind, and that they usually make us aware of what, in external objects, may be useful or adverse to this union…” (Principles, Part II, Art. 3)

These instances can be read as Descartes attempting to correct the errors baked into our ways of speaking and common, vulgar, sense. However, they also paint a picture of how that common sense functions — as a constant union between two (radically distinct, in Descartes’ view) essential substances.

If, with our 20/20 hindsight, we perform the Kantian twist proper to this image, we can say that the world as we have always understood it, inhabited it, studied it, discussed it, and so on, has always been firmly rooted in a hybrid of extension and immediate perception; extended-thinking and immediate extensions, space-time stretching out from a here-now point. What Descartes discovered in his dualism was precisely this synthetic character of experience. Descartes’ rarefied pure geometrical extended substance, as well as his pure thinking substance with no ‘earthly’ intentionality, are two domains that ‘we’ never step into, existing as ‘we’ do in the site of their welded union. Their independent existence is something that can only be posited in theory (in Descartes’ God assisted epistemology this was enough to ensure their independence), and is not the object of any past or previous science (and, if we follow Kant, not an object of any future science either).

So, Descartes has all of the pieces of a proto-Kantian idealism right there. It seems what caused him to miss it was an assumption that a substance could have only (and exactly) one principle attribute, thus dividing the thinking and extended substances into isolated domains (raising serious questions as to their interaction). He also held that time was secondary to movement, and thus not of concern to the thinking substance, but of the extended substance. This meant that as he explored the hard kernel of the Cogito, this indubitable treasure in the seas of skepticism, he failed to notice its strange coincidence with the temporal notion of the present and the knife-edge instant of becoming.

John C. Brady is a student of philosophy and educator situated in Beijing. He gets most of his reading done in traffic jams. He is also a co-editor of this magazine, by way of full disclosure.

Works Cited

Descartes, R., Veitch, J., (2002). The Principles of Philosophy, Blackmask Online.

Descartes, R., Haldane, E., Ross, G. and Chávez-Arvizo, E. (1997). Key philosophical writings. Wordsworth.

Hoffman, P. (2009). Essays on Descartes. Oxford: Oxford University Press.

Skirry, J. (2018). Descartes, Rene: Mind-Body Distinction | Internet Encyclopedia of Philosophy. [online] Available at: [Accessed 23 Apr. 2018].


April 2019


C.S. Peirce on Science and Belief

by Brian Kemple

Stoic Practical Philosophy: A Guide for Life?

by Carl O’Brien

The Irreducible Instant: Descartes’ Thinking Substance

by John C. Brady

Kierkegaardian Love and Resignation

by Timofei Gerber