A Puzzle About Knowledge

A Puzzle About Knowledge

I want to discuss one of the oldest problems in philosophy, not aiming to solve it but with a view to articulating its difficulty. It has a claim to shaping the entire history of Western philosophy, refusing to go away. I mean the problem of a priori knowledge or, as we are apt to say, knowledge by means of pure reason. It is not generally supposed that a posteriori knowledge presents the same problem, though it may present parallel problems. It is supposed that we know about particulars by perceiving them: we have a faculty of perception and it enables us to acquire information about particular things located in space and time. It is not thought impossible to gain such knowledge (putting skepticism aside), because perception makes it possible. It is often maintained that perception works causally, so that perceptual knowledge is simply an effect of external particulars operating on us. This is no doubt oversimplified and not very explanatory, but at least we have a sketch of how empirical knowledge is possible. A further assumption is that we can’t gain knowledge of particulars except by perception (this is the core of empiricism)—in particular, pure reason cannot provide knowledge of particulars. That certainly seems true, but its truth is not as transparent as one might wish. Why is it that reason cannot supply us with knowledge of particulars, if it can supply us with other types of knowledge? Why are we limited to our senses in gaining knowledge of particulars—what is it about particulars that demands such a route to knowledge? Couldn’t we have innate knowledge of particulars, though we have none as things actually stand? Couldn’t we be suddenly blessed with a more direct way of finding out about them? These questions are reasonable enough, but the possibilities they envisage seem remote to the point of non-existence: we must know about particulars by perception and not by our rational faculty alone. Why that is remains obscure, but it does seem that particulars are necessarily known by perceptual means. Just as it is thought that we can’t know about abstract objects except by means of our reason, so it is thought that we can’t know about concrete objects except by means of our senses. In any case, that is not the question I wish to discuss, which is the nature and possibility of a priori knowledge: what kind of knowledge is this, what is its origin, and is it really possible?

            We can take Plato’s treatment of Socrates and the slave boy in the Meno as our paradigm example: the boy is brought by judicious questioning from Socrates to discover Pythagoras’ theorem. On the face of it he relies on his own inner resources in coming to have this knowledge—he doesn’t perceive a whole lot of triangles and then infers the truth of the theorem. So it appears that his gaze is directed inward—at his concepts perhaps. His concepts guide him to the truth not his senses. Does he perceive these concepts with a special inward-directed sense? No, but he has access to them somehow: his knowledge of his concepts leads him to knowledge of the properties of triangles, which are not concepts. So we might suggest that a priori knowledge derives from concepts: we move from truths about concepts to truths about the things they are concepts of. This is by now a very familiar line of thought, but it raises a tricky question: must the conceptual knowledge be gained by looking inwards or can we obtain it by examining someone else’s concepts? What if the slave boy could perceive Socrates’ concepts, or know about them by empirical inference—could he use this as a basis for knowledge of Pythagoras’ theorem? The suggestion does not seem absurd, though admittedly farfetched: surely we could in principle know about the content of another person’s concepts and use this knowledge to infer analytic truths concerning those concepts. This would not be so different from using Socrates’ verbal testimony in order to come to know the theorem—the slave boy could have just been told outright what the theorem is, thereby coming to know it. So is it that such knowledge can be obtained by looking outwards, though it is generally not so obtained? But here an interesting point arises: if the boy had come to know the theorem this way, his knowledge would have been quite different from what it is when he comes to see it for himself. We want to say that he really knows it in the latter case, but only takes it on trust in the former case. He understands it, grasps it, and has insight into its truth. So it is the wayhe knows it that makes the difference—by consulting his own concepts. And doing so in that special way we use when engaged in a priori inquiry (not by looking at our own brain, say). The knowledge has a special meaning for the boy: it strikes him in a certain way. So what is this way—what kind of knowledge is it exactly? When we know something a priori what precisely is our state of mind—what specific type of knowledge do we possess?

            Here Plato came up with a brilliant idea: it is a form of memory knowledge. This is his recollection theory of a priori knowledge—the doctrine of anamnesis. There is nothing like remembering an earlier experience for searing it into your mind—say, what you were thinking and feeling on a certain day 10 years ago (see M. Proust). Compare this with forgetting what happened and being told about it by someone else: you might well come to know the past by means of such testimony, but it is nothing like remembering it yourself “from the inside”. You really know it then, and it would be dreadful if all your memories were erased and replaced by current testimony from other people. Memory knowledge has a special force and vivacity. Thus when Plato says that the slave boy is recollecting what he knew in a previous life he is attributing to the boy a special kind of knowledge that is not at all like hearing it from Socrates right now. We might call this the memory knowledge theory of a priori knowledge: all such knowledge is a kind of memory knowledge—though not of events in this life, but events in a previous life. Problem solved! A posteriori knowledge is newly acquired knowledge courtesy of the senses, while a priori knowledge consists of recollections of knowledge gained in a previous life. We know what recollection is, and recollection is what enables us to have a priori knowledge. There is only one slight snag: we have to believe in a previous life during which we originally acquired the knowledge in question. We have to believe in something like reincarnation (or an eternal disembodied soul). An enthusiast of Plato’s recollection theory might urge that we have here a proof of reincarnation, since no other theory can do justice to the nature of a priori knowledge; and we have the bonus that we can reasonably expect to be reincarnated ourselves. Still, the intellectually pusillanimous among us might balk at such an extravagant theory, wondering how a priori knowledge could have such momentous consequences. Did the slave boy really exist before he was born stuffed with mathematical and other a priori knowledge? Does he now really recollect what he knew then?

            Perhaps we can take the sting out of the theory by updating it. Isn’t genetic transmission a bit like memory? If your parents possessed an item of knowledge genetically, which they pass on to you, isn’t it as if you are recalling what they already knew? Not phenomenologically perhaps, but at least in the sense that a piece of knowledge is being recovered from the past—from a past life in which it was explicitly known. Thus instead of recollection we could postulate genetic transmission—a form of information storage that can be accessed at a later date. Not memory but the DNA. True, recovering such information doesn’t feel much like memory, but this provides a theory of the origin of the knowledge in question, given that it cannot derive from the senses. It doesn’t have the poetry of Plato’s theory, or its power to provide a unique flavor to a priori knowledge, but it does give us an origin story. Socrates was accessing the slave boy’s genes, in effect. The trouble with this theory, which it shares with the recollection theory, is that it is regressive: for how was the first piece of a priori knowledge acquired? It couldn’t be the result of genetic transmission or of recollection. At some point someone had to gain the knowledge in question ab initio—it couldn’t come from a previous life or from antecedently existing genes. So there is no real explanation of the origin of a priori knowledge—no account of how it arose in the first place. How did the slave boy in his previous life come to know Pythagoras’ theorem? Not by recollecting it from a previous life, on pain of infinite regress. How did the first human come to know the theorem? Not by genetic transmission from a previous knower of it. Both theories really leave us exactly where we were, without any account of the origin of a priori knowledge. We are left saying, “We just have it” or “It just pops into our mind”.[1]

            The problem is exacerbated by the fact that this kind of knowledge concerns things external to the mind not psychological matters. It is not like introspective knowledge. Pythagoras’ theorem is about triangles not ideas of triangles. How can we know objective facts by looking inside ourselves? How can we discover truths about objectively existing universals by consulting our inner psychological states? Thus a priori knowledge is puzzling, even paradoxical, and seemingly impossible. The theory of anamnesis, though ingenious, does nothing to resolve this puzzle, merely burying it; and the same is true of the genetic theory. And invoking universals, as in the theory of forms, just accentuates the difficulty, suggesting the existence of a perceptual relation that is false to the facts. We don’t see universals.  A priori knowledge remains a mystery–far more so than a posteriori knowledge. It is knowledge we gain by directing our attention inward, but we have no model of how it works—no explanatory framework. The same knowledge cannot be acquired in any other way, but that fact too is mysterious. The slave boy appears to perform a miraculous feat, conjuring his geometrical knowledge from nowhere, but the same feat is performed by every normal human. Even allowing for recollection of a previous life is inadequate to explain it. It seems to well up from nowhere, unlike a posteriori knowledge, which pours in from outside.[2]C

[1] Russell maintained in The Problems of Philosophy that we have “acquaintance with universals”, analogous to our perceptual acquaintance with particulars, but he gave no account of the nature of this alleged acquaintance, and it is hard to see how the analogy can be sustained (ditto for Godel’s remarks on “intuition”). Clearly we have a faculty of a priori knowledge, but how it works remains as mysterious today as it was in Plato’s time.  

[2] The desire to give reductive accounts of the a priori, such as those proposed by the positivists, is therefore perfectly understandable, if hopelessly implausible. I think Plato’s anamnesis theory is the best theory ever proposed for dealing with the problem, but it runs into insurmountable difficulties. One difficulty I don’t mention in the text is that it is curious that these memories from a previous life show no tendency to degrade and disappear, like ordinary memories. They might even persist unaltered across multiple lifetimes without ever being evoked, if there is no occasion to bring them to light. In addition, we don’t normally react to acquiring a new piece of a prioriknowledge by saying, “Oh yes, I remember that!” I should also note that a priori knowledge brings with it modal knowledge to the effect that the known proposition is a necessary truth. This introduces another layer of epistemic perplexity into the picture. Really the whole thing is utterly baffling. And let’s not forget that a priori knowledge and a posteriori knowledge interpenetrate in our cognitive lives, with the perplexities of the former bleeding into the latter (logic, for example).  


Why Materialism Collapses Into Panpsychism

Why Materialism Collapses Into Panpsychism

Weak materialism is the thesis that mental properties are aspects of entities (states, events) that also have physical properties, as with token identity theories. Strong materialism is the thesis that mental properties are physical properties, as with type identity theories. The physical properties are typically taken to be properties of the brain, specifically neurophysiological properties. It is not generally supposed that mental properties are instantiated outside the brain—so the materialism is taken to be a brain-centered materialism. The materialist doesn’t tend to think that mental properties are instantiated in the world at large simply because they are instantiated in the brain; the materialist is not a panpsychist. But it is difficult to see how materialism can avoid sliding into panpsychism because of the following question: why are brain properties so special that only they are accompanied by mental properties? What restricts the mental to the cerebral, given that the brain is just a physical object like others? If physical properties are co-instantiated with mental properties in the brain, why aren’t they also co-instantiated with them elsewhere? Shouldn’t nature be uniform in this respect? Suppose some Martian scientists, composed of exotic material unknown to us and without brains like ours, were to visit earth and investigate terrestrial brains, discovering that they house mental properties as well as physical. Wouldn’t it be a reasonable induction on their part to infer that other matter on earth must also be correlated with mental properties? After all, there is nothing special about the matter of the brain—it is just complexes of molecules—so we would expect that matter in general would show the same correlations. If mind supervenes on matter in the brain, why shouldn’t it supervene on matter in other locations? There is nothing unique about matter in the brain—that is the whole point of materialism—so matter should exemplify mind everywhere. To be more specific, if it is electrical activity in the brain that is key to the existence of mind, then electricity should be associated with mind wherever it exists—which is pretty much everywhere. Hence materialism collapses into panpsychism.

            It is true that this seems odd from a human perspective, because we don’t normally suppose that mind exists virtually everywhere—that is not our commonsense conception of the world. We feel it only in ourselves and infer it only for beings similar to us. But consider this analogy: we have discovered that heat is molecular motion, but molecular motion exists almost everywhere, so heat does too, despite that being contrary to what we normally think. Even cold objects have heat in them! Only at absolute zero—a rare state of affairs—does heat disappear. It isn’t that heat exists in boiling water but not in rainwater despite molecular motion in both—that would be contrary to the uniformity of nature—but rather that our common ways of talking are in error. The thermal and the molecular-dynamic are indissolubly connected, contrary to initial appearances (we only feel heat in certain things as a result of the peculiarities of our contingent senses). Similarly, we only feel mind in the case of brains, but since mind is an aspect of matter in brains, it is also an aspect of matter elsewhere—by the uniformity of nature. It isn’t that mind miraculously disappears from matter once matter leaves the confines of the head! That matter came from elsewhere and will go elsewhere, so it had mind in it from the beginning and will go on having it. Another analogy: suppose we discovered that liquid water is H2O but had never thought about ice being water (we have never seen the transition from solid to liquid water); then we discover that ice is also H2O. It would be wrong to declare that not all H2O is water because ice is H2O but not water; rather, we have discovered, contrary to our initial impressions, that water is more widespread than we supposed, existing also in the frozen polar-regions. In the same way the panpsychist maintains that the co-existence of mind and matter in the brain is not an isolated freak of nature; rather, it is a universal fact of nature. If matter can be mind in the brain, then it must be mind elsewhere too. Thus materialism and panpsychism necessarily go together.

            This is not a problem for materialism per se; it just turns out that materialism implies that mind is spread more widely than is commonly supposed. It turns out that it is spread as widely as electricity, this being the important physical property in the case of the brain. We already knew that mind has two locations in the human body—the brain in the skull and the enteric nervous system (the “bowel brain”).[1] Not exactly pan- perhaps, but at least bi-: so we should be prepared to accept that the mind is spread even more widely. Nor is this at all inconsistent with materialism: materialism says that the mind is in the matter of the brain; panpsychism says there is mind wherever there is matter. Everywhere that mind is matter is there too, which is the essence of materialism (either weak or strong). Nothing dualistic is implied by panpsychism. It is just that the materialist might be surprised to find himself a proponent of panpsychism—that wasn’t quite what he signed up for. But if you insist that the essence of mind is matter, don’t be surprised if mind turns up everywhere that matter is. The only way out of this is to claim that some kinds of matter are special, capable of producing mind while other matter is mentally inert; but that is to give up on the materialist doctrine—mind is not spooky matter! And don’t start waving your hands about the brain’s complex organization: that is not what the materialist is maintaining, but rather that mental properties are tied to matter as such—not its abstract organization (whatever that is exactly). As some wit once remarked, British Rail is complex and organized too, but it isn’t conscious! No, the materialist may as well acknowledge that panpsychism comes with the territory—just as pan-thermalism does.

            What does not imply panpsychism is dualism: if the mind has nothing essentially to do with the brain, then there is no argument showing that it must be found associated with other physical things—for it may exist in isolation from the matter of the brain. Mind could be annexed to other physical things, according to a dualist, and so is not inextricably connected to specific sorts of matter. Nor is it an inevitable accompaniment of matter, or matter of it. Similarly, if heat had nothing to do with molecular motion, there would be no argument showing that heat exists wherever there is molecular motion. So dualism does not imply panpsychism. Nor does a view that asserts the existence of a special mind-producing property in the brain, perhaps unknown, that is not found elsewhere.[2] So long as this property is found only in the brain, there is no slide into panpsychism, since it is not matter as such that is the determining factor. Functionalism could go either way, depending on how it is formulated: if it is just the existence of causal inputs and outputs that matters, we get a similar slide; but if we try to beef it up with something conceptually richer, the answer will depend on how widely that beefed up functional property is distributed (biological function will certainly involve a good deal of spread). If functionalism is taken as a species of materialism, then it will surely tilt in a panpsychist direction. So actually it is hard to avoid the panpsychist spread in anything short of something pretty drastically dualist, i.e. something that links mind uniquely to something far more restricted than matter (or electricity, or cellular structure, or functionality). It is not so much that materialism (and like doctrines) are too materialistic; it is more that they are too mentalistic—for they find mind spread far and wide and in the most unlikely of places. Materialism makes the whole physical world mental.

            It is a good question how widely the net of panpsychism should be cast. People often say that atoms are to be construed as having mental properties (or “proto-mental” properties blah-blah-blah), as if this demonstrated the boldness of the doctrine. But that can hardly be the logical end of the line. What about electrons and protons? What about quarks and strings? Does panpsychism apply to points in an electromagnetic field? What about points in empty space? What about the laws of nature—are they also partly mental? And what about time—do instants have a mental dimension too? Mental events and processes occur in time, so is their temporal nature a result of the mental aspect of time itself? Or is it just plain old non-mental time transposed to the mind? But then won’t it be left unexplained how psychological time can exist? The logic of panpsychism is extremely far-reaching, making everything in the world a repository of the mental. Do numbers have a mental nature too? Are geometrical shapes mentally endowed? In any case, depending on how far we push panpsychism, materialism will require us to accept some version of it. Just by identifying pain with C-fiber firing we could end up claiming that space and time are bubbling with mentality! For C-fibers are just matter in one configuration, and it’s the matter that counts not its configuration (how could that—a matter of mere geometry—be what pain consists in?).[3] If mind is associated essentially with this kind of matter, the kind in the brain, then it must be associated with all matter, on pain of denying the uniformity of nature (as well as invoking an ad hoc stipulation). Thus materialism turns out to presuppose panpsychism precisely because it seeks the basis of mind in matter. Being located in the brain does not alter the intrinsic nature of matter, so materialism is committed to the idea that matter as such is the basis of mind—and matter is everywhere. Only an arbitrary stipulation can prevent mind from cropping up wherever matter does. Fine, if you are OK with panpsychism, but not otherwise.

            Note too that the same slide occurs on a less macro scale once we remember that not all of the brain is associated with mentality. Only some neural clusters are correlated with mental states, not all, yet they all share the same basic morphology and physiology. So it can’t be physical cell-type that makes the difference. The natural response is to declare that all the neurons in the brain are at some level endowed with mental properties, or else our materialism will be unacceptably arbitrary—what we could call pan-neural-psychism. But then the question will be why stop there—why not include heart and kidney cells? We have discovered that all the cells of the body have mental properties, given that we know that some of them do. The point is that it is impossible to stop the spread of the mind once we accept a materialist view of the mind as we locally conceive it. If C-fibers suffice for pain, or necessarily have pain as an aspect, then why don’t other fibers, whether in the brain or outside it? And why does it have to be fibers and not some other physical feature? To repeat, there is nothing special about the matter of the brain, even (especially) according to the materialist, so we can’t avoid inferring that mind must be present wherever there is matter. Universal mentalism follows from local materialism (not deductively, of course, but as a matter of overall theory). That is surprising perhaps, but not necessarily disastrous. From the point of view of the panpsychist, it turns out that grounds for believing materialism are grounds for believing panpsychism: if materialism is true, then panpsychism has to be true too, on pain of an arbitrary stipulation.[4]

[1] See The Second Brain, by Michael Gershon (1998).

[2] I discuss such a view in “Can we solve the Mind-Body Problem?” (1989).

[3] It is generally accepted that it is not the color or location or size of C-fibers that matters to their being pain, so why should their shape make all the difference? It is the matter itself that is supposed to constitute the pain—but then why not pan-painism? What’s so special about neurons? They are just particularly stringy biological tissue, which itself is made of non-biological components.

[4] If the mental is the physical, then the physical is the mental, by the symmetry of identity. That is, if mental states are identical to certain physical states, then those states are identical to mental states, and hence the physical states necessarily imply the mental states (C-fibers are pain in every possible world). The point then is that there is nothing special about those physical states, so that panpsychism is the only reasonable position. They are not states with a special magical glow. 


Phenomenology of A Priori Knowledge

Phenomenology of A Priori Knowledge

What is it like to know something a priori? How is it subjectively to know (say) that nothing can both be and not be, or that 2 + 2 = 4? The traditional definition of a posteriori knowledge has it that such knowledge is “dependent on experience”, while a priori knowledge is knowledge that is not dependent on experience. But what is it to know something “by experience”? What properties of experience are relevant to determining the status of an item of knowledge as a posteriori? And how do these properties affect the phenomenology? Evidently an experience is a conscious state, so we would expect that the phenomenology of consciousness would play a role, i.e. what an experience is qua conscious state. It is a specific type of conscious state—the type associated with the operation of the senses. We might then suppose that subjectivity is important: the property of being graspable only from a specific point of view. Then we could say that a posteriori knowledge is knowledge based on evidence that can only be grasped from a specific point of view, i.e. subjective evidence. By contrast a priori knowledge is not based on such subjective evidence—perhaps it is based on objective evidence, i.e. the kind that can be grasped from any point of view. That is certainly an interesting way to formulate the distinction, though it seems rather theory-laden as a criterion of the a priori. Experience is subjective in this sense, but being so seems irrelevant to the distinction in question; and it isn’t clear how it affects the phenomenology of the two types of knowledge. Is it that a posterioriknowledge can only be grasped by someone who shares the type of experience in question, while a prioriknowledge can be grasped irrespective of sharing experience types? But how does that affect what it is like to be the subject of the two types of knowledge? How does the phenomenology differ? Alternatively, we might interpret “experience” to mean “as a result of past experience”, where we use the notion of what has been learned in the past. Then the idea will be that a posteriori knowledge is knowledge acquired by evidence gathered in the past and now stored in memory, while a priori knowledge is not so acquired—it doesn’t rely on “past experience”. This has nothing to do with subjectivity and points of view, and it is a perfectly acceptable use of the word “experience” (“I’ve had a lot experience bird watching”). But it is silent on the question of phenomenology: it says nothing about how it seems to the subject to have the two types of knowledge. Maybe it has implications for phenomenology, but it isn’t a phenomenological description. So is it that there is no phenomenological distinction here—no difference of seeming? Is it that the subject cannot tell the difference between the two sorts of knowledge by means of introspective awareness? There is no phenomenological divide between knowing something a priori and knowing something a posteriori—though there is indeed a distinction between the two types of knowledge. Does the traditional epistemic distinction have a phenomenological counterpart?  

            We do better to change tack and consider intentionality. One of the characteristics of experience is that it has intentionality—experiences have objects (not necessarily existent). When a subject acquires a piece of a posterioriknowledge she has an experience of an object, as it might be a bird she is seeing. But, it may be thought, a prioriknowledge, not being experience-based, has no such object—it is objectless. This would certainly be reflected in the phenomenology: in one case experience of an object, in the other no experience of an object, because no objectat all. Intuitively, one acquires empirical knowledge by experiencing objects, but rational knowledge does not involve experiencing any object—it comes from “pure reason”. This differentia seems to fit paradigm cases of a priori knowledge quite nicely: our logical knowledge is not based on any apprehension of objects but (as we obscurely say) arises from our grasp of concepts; and our knowledge of analytic truths comes from senses not references. There is nothing like seeing a bird involved, because there is nothing experiential that is involved—and hence no experienced object. Thus a priori knowledge is objectless knowledge, and this is a phenomenological fact about it. It has a distinctive type of intentionality—involving concepts not objects, to put it simply.[1] Alternatively, we could say that a priori knowledge is directed to universals while a posteriori knowledge is directed to particulars: abstract not concrete, general not specific. 

            But this suggestion runs into a problem with mathematical knowledge. According to Platonism, numbers are objects, so isn’t knowledge of mathematics a posteriori according to the suggestion that an intentionality of objects is necessary and sufficient to make something a case of the a posteriori? Other metaphysical theories of mathematics would not have this consequence—formalism, fictionalism, logicism, maybe intuitionism—but shouldn’t mathematics be a priori even under Platonism? Actually that is not so obvious on reflection: suppose cosmologists discover a peculiar kind of light matter in a remote section of the universe that gives off a faint kind of energy that is perceptible by the human nervous system—and this is associated with mathematical knowledge. Wouldn’t we then say that it has turned out that mathematical knowledge is a posteriori? There is a type of experience of mathematical objects, barely perceptible, that gives rise to mathematical knowledge, thus rendering it a posteriori. But there is also this question: even if there are mathematical objects, are they part of the phenomenology of mathematics? That is far from clear: does it seem to us that Platonism is true? Is that a datum of consciousness? And what notion of object is in play here? It is certainly not the notion of material particulars in space, or even mental particulars in non-space. We don’t feel ourselves confronted by numbers, as we feel confronted by physical objects—faced with them, assailed by them. To call numbers objects seems like a stretch, precisely because that is not true to the phenomenology: it doesn’t seem to us that numbers are at a certain distance from us and each other, or that there is perceptual constancy with respect to our perception of numbers, or that there is a type-token distinction for numbers. Talk of objects here feels theoretical and tendentious not natural and intuitive. Nor does anyone ever argue that Platonism must be true because numbers seem like objects. Nor is it that our mathematical knowledge is produced by anything deserving the name of experience, so we don’t have it as a result of experiences of objects. So we can reasonably say that mathematical knowledge is not object-directed from a phenomenological point of view; it is nothing like our knowledge of material things in this respect (this is why non-Platonist theories don’t immediately strike us as false to our lived experience with mathematics). It is true that mathematical knowledge is problematic—notoriously, it isn’t causally explicable—but it is not obligatory to describe it as phenomenologically object-directed. And even if it were, we could always amend our criterion to say that a priori knowledge is not based on experience of particulars—concrete spatiotemporal entities—but rather on the apprehension of abstract entities (non-particulars).

            There is a further fact about sensory experience that distinguishes the a priori from the a posteriori, namely the role of the sense organs. It is part of the phenomenology of empirical knowledge that we are aware of our sense organs, and hence our body. I know, for example, that my current experience of a bird is visual and that my eyes are involved in having such experience. When acquiring knowledge empirically I am aware that my senses are operating in a certain way—my eyes and head are moving, for instance. This is a part of the phenomenology—not just external objects of experience but also the object that is my body, specifically the physical sense organs. But in the case of a priori knowledge there is no such awareness of my body—I am not aware of a bodily sense organ responsible for picking up information about the objects of my a priori knowledge. I am not aware of a part of my body vouchsafing to me mathematical knowledge, say. Of course, the brain is involved in having such knowledge, but the brain is not a phenomenological component of acquiring that knowledge—there is no bodily sense organ geared to the mathematical world. So there is a double independence from objects: no external object of awareness, and no bodily object producing awareness. In gaining empirical knowledge I have awareness of external objects and I also have awareness of my own body as a mediator of knowledge, but neither is true of a priori knowledge. In this sense the phenomenology of a priori knowledge is disembodied as well as objectless.

            I would describe a priori knowledge as phenomenologically empty: it is not experiential or object-directed or body involving. By contrast a posteriori knowledge is full: replete with objects and the body and subjective experience. Sartre might say that the former is a phenomenology of Nothingness while the latter is a phenomenology of Being. Of course, a priori knowledge has intentional objects—concepts, universals, abstract entities—but these don’t saturate consciousness in the way the components of a posteriori knowledge do. I am now aware of my visual experience, its concrete objects, and the body that enables me to have experience of objects: but I am not likewise aware of such things when I think about logic or numbers or analytic propositions—I am simply aware of certain truths. I am aware of the apparatus of knowledge in the former case, but not in the latter. Thus a priori knowledge seems more mysterious to me, because I can’t apprehend its machinery—what makes it possible. From a phenomenological point of view, it seems like a deliverance from on high—a kind of miracle. I can know that 2 + 2 = 4! How I know this, I don’t know—but I can know what makes my knowledge of my immediate environment possible, viz. experience, perceived objects, and my sense organs. This is the phenomenological upshot of the traditional way of marking the distinction between the a priori and the a posteriori: it arises from the nature of sense experience as a phenomenological category. The phenomenology of a posteriori knowledge is the phenomenology of experience as object-directed and body-mediated; the phenomenology of a priori knowledge is absent these components, presenting itself rather as objectless and disembodied. The body and the perceived world are phenomenologically irrelevant to the etiology of a priori knowledge. It appears to transcend such things.[2]

[1] Of course, both sorts of knowledge have objects—either concepts or objects—it is just that concepts are not objects. This is the familiar ambiguity of “object” as between a type of entity and being a target of a mental state (an “object of thought”). 

[2] I am not saying it does transcend the world of material particulars—we need a body and brain to have a prioriknowledge—but as a matter of phenomenology these things don’t enter the picture. Hence a priori knowledge has a curious thinness and impalpability. It is characterized by absence—what is not suffused by experience, objects, and the body.