Huemer’s Theory of Perception: Analysis and Objections


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By Ethan Rubin

In his book Skepticism and the Veil of Perception, Michael Huemer lays out an account of perception that supports a version of direct realism. He states two main theses: that perception is direct awareness of external reality, and that it leads to non-inferential knowledge of that reality. The second claim requires that the first be adequately defended, which is the goal of Chapter IV in particular. In this paper, I intend to map out Huemer’s argument and assess its strengths and weaknesses; each section explaining a key point in his theory. After it has been explained, I will bring up objections to certain aspects of the theory as they appear in the text, and consider the most promising defenses against them.

Awareness requires an apprehension and a non-accidental correspondence between apprehension and object

Huemer’s first task is to clarify his claim regarding direct awareness. He uses the term “awareness” in a technical sense, as the relation between one who is aware and the object of which he is aware. This establishes that one must be aware of something – if there is no object present, then there cannot be actual awareness. This excludes cases of hallucination from awareness.
Huemer claims that awareness must include a state of apprehension. He defines this term as a mental state of assertive representation. Not all mental states, he claims, have representational content (the example he uses is the sensation of tickling). Thoughts, desires, and perceptions, however, do have representation content, but represent their objects in different ways. Apprehensions are characterized by actualized representation, meaning that they represent their objects as being the case. He calls this characteristic the apprehension’s assertiveness. Apprehension, therefore, must represent an object as actually existing, as opposed to representing it as a possibility or command.
Huemer also requires that the apprehension of an object at least roughly correspond to the nature of the object. He calls this “satisfying the content of the representation”. By accepting the possibility of some deviation, he allows for certain common illusions, such as the appearance of a bent stick in the water. The correspondence between the awareness of the stick and the stick itself is close enough that the viewer can correctly identify it and describe it with reasonable accuracy. If the stick were perceived as a different object, such as a green kitten, there would not be enough correspondence between the apprehension and its object to consider the relationship awareness.
Huemer also stipulates that the correspondence between apprehension and its object cannot be accidental, stating that “the apprehension must have been formed in such a way, and under such conditions, as to make its correspondence with reality probable”. For instance, if one were to hallucinate a particular scene while that scene coincidentally occurs elsewhere, he or she cannot be said to be aware of that event. Note that this scenario is unsatisfactory even if the correspondence is perfect. This sort of example highlights two problems with accidental correspondence: the apprehension is not of the true object, and the observer does not have sufficient reasons to believe that it is.

Direct awareness is unmediated

Huemer’s theory hinges on the difference between direct and indirect awareness. In indirect awareness, he states, one’s awareness is of x, but only by means of his awareness of something else. When driving a car, for instance, one is aware of the amount of gas left in the tank. This awareness is indirect because it is based on the driver’s awareness of the indicator needle on the dashboard, which he or she expects to reflect the amount of gas in the tank. Direct awareness, by contrast, is unmediated by such a secondary awareness. In order to be indirectly aware of something, one must first have a direct awareness of something else. Indirect awareness is a causal and logical relationship – one is led to reach the second awareness by a logical connection between it and the first. As such, it transmits the authority of the direct awareness to the indirect. This requires that direct awareness refer reliably to external reality.
Three Elements of Perception: a Perceptual Experience, an Object of Perception and a Causal Relationship

Having laid the foundation for his theory, Huemer turns to an analysis of perception. He identifies three major components of perception, each structured similarly to awareness. First, there is a purely internal mental state, which he calls the perceptual experience. Second, there is an object of perception, which is external and at least roughly satisfies the content of the experience. Third, there is a causal relationship between the two. The absence of any of these elements disqualifies the event in question from being perception.
The perceptual experience differs from perception as a whole in that it does not necessarily entail an external object. Because the experience is internal, it is not necessary that it be accompanied by an object. This is how Huemer accounts for hallucination. A perceptual experience does occur in hallucination, but there is no external object present. The perceptual experience still occurs because on an internal level, hallucinations are indistinguishable from genuine perception. This implies that they share a common mental state and that internal experiences should be recognized as separate from external objects of perception.
As in awareness, the object of perception must meet a standard of resemblance to the content of the perceptual experience. This standard allows for some discrepancies, but only to a point: when the experience represents the object as being fundamentally different than it is in reality, it can no longer be called perception. The content satisfaction criterion is present in other representations as well. A painting, for example, can diverge from its subject’s appearance to a certain extent, but at some point it can no longer be considered a painting of that subject.
Huemer recognizes another sort of perceptual error that must be explained. It is possible to perceive an object in a manner that corresponds to the actual nature of the object and still mistake it for something other than it is. For instance, one might see a coil of rope and mistake it for a snake. To resolve this, Huemer introduces the notion of primary versus secondary perception. In secondary perception, the object is perceived by virtue of another perception: perceiving the coil of rope or the snake involves the same primary perception, as the same image is seen in both cases. The difference lies in the secondary perception, in which that same image is taken to be of different objects. The coil of rope is therefore seen as a snake. This sort of error is only admissible to a point, much like the case of illusion in primary perceptions. A mistaken secondary perception only arises from a primary perception that has a reasonable resemblance to the secondary perception; a coil of rope does have some visual resemblance to a snake. If someone were to mistake the coil of rope for a bear, there is a more serious error at work.
Finally, there must be a connection between the internal experience and the external object. Huemer proposes that the object must have caused the perceptual experience if the experience is of the object, thereby excluding coincidental correspondence. This causal relationship also must be direct; Huemer excludes what he calls deviant causal chains. If you were to have an accurate perceptual experience, but only because someone with the appropriate knowledge implanted the perception in your mind, it is true in a sense that the presence of the object caused the perceptual experience. The causal chain, however, has an intermediary link and is not perception.

Three Elements of Perceptual Experience: Sensory Qualia, Representational Content and Forcefulness

Huemer proceeds to subdivide the first component of perception, perceptual experience, into three features. He claims that a perceptual experience always has sensory qualia, which are defined as “what it is like” to have the experience. It also must have representational content, and that content must have forcefulness – the characteristic of seeming present and real. These components are notably similar to those of awareness.
According to Huemer, all experiences are accompanied by qualia; sensory qualia are those that correspond to perceptual experiences, as opposed to emotional or imaginative experiences. Qualia exist over and above representational contents, and are ineffable in that they cannot be explained to someone who has never had a comparable experience. Someone who was born deaf, for example, cannot understand what it is like to hear. This ineffability is not mystical or metaphysical, nor does it apply to common experiences and normal perceivers – two people with unimpaired hearing can describe sounds to each other effectively.
On Huemer’s theory, perceptual experiences also involve representational content. He explains that “things appearing to be a certain way is not some further consequence of your experience; things appear a certain way by virtue of your having the perceptual experience itself”. This assertion challenges theories of perception that ascribe appearances to an act of interpretation that takes place after the perceptual experience. For Huemer, representational content is an essential part of the experience. Consider the bent stick illusion: the stick is not interpreted as being bent. On the contrary, the experience represents it as being bent and the viewer interprets this appearance as an illusion. If interpretation determined the object’s appearance, the illusion would only occur if the perceiver believes the stick is bent. But no normal perceiver would argue that the stick is bent, despite the fact that it appears to be so. The temptation to say the stick is bent is based on how the stick looks. Hence, this appearance must be an integral part of perceptual experience and independent from interpretation.
Representational content is crucial to learning because it represents the object of perception as something that actually exists. This evokes Huemer’s description of assertiveness – a perceptual experience represents its content as actualized, or as being the case. It follows is that representational content is propositional. A perceptual experience expresses the proposition “it is true that object X is before me,” which is a precondition for reaching conclusions regarding that object. This proposition leans on another, namely that “it is true that my perceptual experience portrays the external world with satisfactory accuracy.”
Huemer is careful to point out, however, that propositional content does not necessarily imply conceptual content. Conceptual content, unlike representational content, is not an intrinsic property of an experience. A perception is the same whether the perceiver has concepts for all the objects, for only some of them, or for none at all. In fact, we cannot have enough concepts to address the nuance and variability of experience. For example, the concept “red” is insufficient to differentiate between the many shades of red that we perceive as being distinct. The use of demonstratives can address them because it has a pointing function, saying “I see it is that, or thus and so.” This pointing does have propositional content, i.e. “it is true that I am perceiving that object, which is that way,” but the perceiver does not have concepts for “that object” or “that way.”
By rejecting the necessity of conceptual content, Huemer is not rejecting the possibility of conceptual content. He agrees that conceptual content exists and is capable of affecting perceptual experience. His point is that perceptual experiences can occur without conceptual content, implying that conceptual content is not an intrinsic or essential component of perception. He refers here to Wittgenstein’s ambiguous “duck-rabbit”. He admits that if one has the concepts of duck and of rabbit, the picture has the conceptual content of both duck and rabbit. If one only has the concept of duck, however, the picture will not have the conceptual content of rabbit. If the viewer does not have either concept, the picture has no conceptual content, but the viewer still has a perceptual experience. Perceptual experience can be altered by the inclusion of concepts, but it does not have to be conceptualized and does not depend on conceptual content.
The propositional, assertive nature of perceptual experience leads Huemer to his discussion of forcefulness. He begins by asking how imagining and perceiving differ. The two can have the same content – if one imagines a duck or sees one, the content is a duck in both cases. One possible difference is that a perceptual experience has more detail and precision than an imaginative one, but this difference can be overcome by focus and training, or by a photographic memory. Detail, therefore, cannot be the fundamental distinguishing factor between the two. No one confuses imagining with perceiving, because the latter represents its content as being actualized, whereas the former does not. Huemer calls this difference forcefulness. This should not be confused with Hume’s concept of vivacity, which is more like the difference in detail that Huemer considers and discards. Hume’s vivacity is merely a question of faintness or vividness, while Huemer’s forcefulness contains an element of being present.

Why This Is Direct Realism

One could support the majority of Huemer’s claims and still be a proponent of indirect realism by arguing that perception makes us aware of mental states that become knowledge by a secondary, non-perceptual process. Huemer, however, insists that we are directly aware of more than just mental states and that his theory of perception satisfies his definition of direct awareness. As he explained earlier, awareness consists of an apprehension, or assertive mental representation. His description of perceptual experiences accords with the definition of apprehension: perceptual experiences have forcefulness (making them assertive), qualia (making them mental) and representational content.
Huemer’s theory fits the definition of direct awareness by stipulating a causal connection between the perceptual experience and the object of perception. Saying that awareness must be of something is equivalent to saying that it must be caused by its object. Therefore, asking what a perception is awareness of is also asking what causes the perceptual experience. Huemer maintains that only physical facts can satisfy the contents of perception and therefore must be their source. It makes no sense to say that mental states cause perceptual experiences because the contents of perceptual experiences are not present in mental states; mental states do not have the properties, such as shape, color and texture, which form the contents of perceptual experiences. He rejects brain states for the same reason; normal perceivers do not have the perceptual experience of synapses firing. Perceptual experience cannot be caused by mental or brain states because they do not bear resemblance to its contents. The visual (or auditory, olfactory, etc.) experience is first and foremost of the object one is experiencing.
Based on these arguments, Huemer concludes that perceptual experiences are directly caused by physical objects that have the attributes that the experience represents them as having. According to Huemer, it follows that “In the primary sense of ‘aware’…we are directly aware of the fact that there are objects with those colors and shapes”. That is, because our perceptual experiences are direct, the knowledge we gain from them is also direct. He attributes the opposing argument, which holds that knowledge of external objects is indirect because it is based on perceptual experiences, to a confusion between the object of awareness and the vehicle of awareness. This is clarified by the following analogy: one must use an axe to chop wood. He is not, however, chopping the axe, but is chopping the wood by using the axe – the wood is the object of the chopping and the axe is the vehicle, or means, of the chopping. Applying this to perception, the perceptual experience is the metaphorical axe that “chops” external objects. We cannot perceive an object without having a perceptual experience that represents it to us, but that experience is only a tool. The awareness is directly of the objects by means of perception, just as wood is chopped by the man using the axe.
To put it another way, we do not perceive our perceptual experiences. If awareness were based on perceptual experience, we would have awareness of external objects by perceiving experiences, but this is not the case. We perceive external objects by having perceptual experiences, not by perceiving perceptual experiences. The same goes for awareness: our awareness does not arise from being aware of an apprehension, but simply by having an apprehension. Perceiving perceptual experience or being aware of apprehensions requires a second order act in which one turns his attention to processes that occur whether or not he reflects on them. This is an extra step, requiring introspection and deliberate effort, not a constitutive element of perception.

Objection 1: How to Recognize Knowledge

Huemer fails to account for a practical problem in his theory. He admits that one must be capable of handling certain mishaps that tend to befall perception from time to time, but does not make any conclusive statements about how this is to be done. Hallucination and correctness by coincidence are problems that he needs to address more thoroughly.
His definition of awareness stipulates that it is a relationship and therefore must be of something. This condition excludes hallucination from awareness because one cannot have a relationship with something that does not exist. There is an obvious external difference between awareness and hallucination that makes them fundamentally distinct, but this does not suit Huemer’s project. Since he is describing awareness phenomenologically, from the inside, he must account for how the perceiver himself understands his mental states. It makes sense to say that one cannot have a relation with a nonexistent object, but if he feels like he is in a state of awareness and cannot differentiate between his hallucination and a real object, it is unclear how the distinction is salient.
Huemer does admit that there is some irregularity in perception. His position is that we have knowledge via perception, which is not the same as saying that all perceptual experiences constitute knowledge. Even so, the fact that we cannot tell the difference between awareness and hallucination is problematic because it impedes knowing which experiences lead to knowledge. Huemer could argue that one is inclined to judge that an experience is hallucinatory when it does not accord with his expectations of reality, correcting for errors in perception after the fact, but this raises two problems. First, it is only a plausible solution if the hallucination is recognizably outlandish and depicts an “object” whose existence is improbable enough to make the hallucinator disregard his perceptual experience – remember that the mental state is the same in both perception and hallucination, the only difference being the presence or absence of the object. It is not implausible that one could have a mundane hallucination of an unsurprising object, or even an existing object that he had accurately perceived before. There would be no cause for suspicion and he would accept the hallucination as awareness. Second, inserting an act of judgment in the case of hallucination would leave Huemer open to the objection that such an act of judgment takes place in other cases as well. The mental state involved in hallucination is the same as that involved in perception – if hallucination is subject to review over and above the experience itself, normal perception must be subject to the same process of judgment. The inclusion of judgment in this context begins to stray dangerously close to indirect realism, which Huemer wants to avoid.
Similar arguments can be made regarding Huemer’s demand for a direct causal connection between apprehension or perceptual experience and the object represented. The distinction he makes between an acceptable connection and an accidental or deviant one is once again external rather than internal. One example he uses of accidental correctness is that of calling a friend. *After the phone rings multiple times, one may conclude that the friend is not home. However, one could unknowingly have dialed the wrong number and not actually tested whether the friend was home or not. The conclusion may still be correct by coincidence and, by asking the friend whether he was home at the time, he can come to know that his conclusion was correct, and yet this does not qualify as awareness. The reasons why it cannot be awareness are clear to the third party reading the example, but the person in the example has no access to those reasons. There is no way for him to verify the relationship between his experience and his apparent knowledge.
A deviant causal chain presents the same problem. In Huemer’s example, someone has a perceptual experience of a cup. A scientist has implanted an exact copy of the perceptual experience caused by the cup in the would-be observer’s brain, with the result that the experience does correspond with the object it represents. The cup caused a perceptual experience in that it caused the scientist to implant that specific experience, but the subject cannot be said to have perceived the cup because the causal connection is too far removed. The reader has no trouble understanding why this is not perception, but the deceived character in the example cannot reach the same understanding. In light of these examples, causal connection seems like a dubious criterion. If one cannot distinguish between the causal chain of a genuine apprehension and that of an “apprehension” based on coincidence or deception, then one cannot use causal connection as a standard for accepting or rejecting his own experiences. It seems that Huemer owes us a better solution to the internal problems of recognizing knowledge.

Objection 2: Where to Draw the Lines

Huemer is satisfied with some ambiguities that should make the reader wary. By making provisions for rough correspondence between a perceptual experience and its object but disqualifying those that do not correspond enough, he commits himself to a spectrum that covers the entire range between exact resemblance and complete disjunction. At some point in this spectrum, experience ceases to be perception and becomes hallucination. Most readers would ask Huemer to indicate where this point lies. It is easy to make an absurd example that highlights the difference between the extremes – compare seeing a stick in water that appears bent and seeing a stick in the water that appears to be a green kitten. The closer we come to the middle of the spectrum, however, the more difficult it is to distinguish – compare seeing a stick in water that appears bent and seeing a stick in water that appears slightly more bent than the optical phenomenon accounts for. How much more bent can the stick appear before one is no longer perceiving the stick?
Huemer might respond that the spectrum is ambiguous by nature and that distinctions must address the case at hand and its context. This may be true, but there must be some general standard in place by which to make the distinctions in each case. Even if it highlights a vague section of the spectrum as the area in which experience begins to lose legitimacy, there should be some kind of criterion. Saying that acceptable cases “roughly satisfy” the representation and unacceptable ones “diverge radically” tells us very little about what qualifies and what does not. Despite the intrinsic fuzziness of the correspondence condition, Huemer could stand to do a better job of defining the spectrum and identifying the boundaries of perception.
He falls prey to the same pitfall in describing the deviant causal chain. He states that the causal connection between perceptual experience and its object must be close enough that the object directly causes the experience, but “directly” does not specify what qualifies. I had to suppress laughter upon reading the sentence, “I shan’t enter into the question of how exactly one might define ‘deviant causal chain’….” Huemer identifies direct causal connection as an essential component of perception, in whose absence perception cannot occur at all. A deviant causal chain precludes direct causal connection, meaning that the experience in question is not perception. Seeing as this is central to his theory, how can Huemer justify glossing over what a deviant causal chain is? He is obligated to specify what is deviant and what is direct – saying that the deviant chain is “convoluted and abnormal” replaces “deviant” with synonyms and does nothing to pin down what it means.
This problem could also be addressed by appealing to case-by-case decisions. For the most part, what Huemer means by a deviant causal chain is understood despite the absence of a precise definition. The objection incites a sort of Wittgensteinian reply, as in “you know what I mean, this is perfectly clear to you when the case arises.” This solution works better here than it would for the previous objection. The resemblance condition must allow for a range of illusions and imprecisions, whereas the causal connection is a stricter requirement. The presence of virtually any intermediate cause between the experience and its object is sufficient to disqualify the causal chain, which makes it much easier to consider a case and decide that the relationship is not direct.

Objection 3: Unnecessary Components of Perceptual Experience

Huemer’s analysis of perceptual experience does a good job of analyzing the various features that are present, but his insistence that they are separate entities may be misled. He claims that sensory qualia, representational content and forcefulness are all different components of a perceptual experience. Upon consideration, however, it seems that they are inextricably linked and cannot be disjoined. Rather than split up perceptual experience into three elements, it would be more accurate to speak of it as a single entity with features that account for its particular nature. Forcefulness, for instance, can be considered an element of representational content, as it is essential to and inseparable from perceptual experience. Without forcefulness, the experience is no longer a perceptual one: it is an imagination or a memory. This is because forcefulness is a feature of how the object of perception is represented to the perceiver, namely as being present. But Huemer had posited assertiveness long before he introduced forcefulness. Assertiveness is an essential aspect of representational content and lends immediacy to experience; seeing as assertiveness does the same work as forcefulness, there is no reason to tack forcefulness onto the end of the theory.
Sensory qualia are the most dubious of the components Huemer identifies. He begins his discussion with the disclaimer that their ineffability is not mysterious and must not be mistaken for “a resort to obscurantism and mysticism”. This is not, however, the strongest objection against qualia. Even having accepted their ineffability as a characteristic of experience, the existence of qualia still seems superfluous. Because perceptual experiences are internal, as Huemer states, qualia could just as easily be an intrinsic characteristic of perceptual experiences without being a separate class of mental entities.
Huemer’s point is that the representational content and the object that satisfies it are the same, but because there is some difference between the object and the experience, some “what it is like” to experience the object, qualia must exist. The function he attributes to qualia, however, can easily be included in the notion of representational content. Representational content represents an object as being a certain way to the perceiver. This is an aspect of his internal mental state and as such has the same ineffability as qualia; it represents the object as being “this way.” As it represents the object to the perceiver, the perceiver experiences the representational content. How is this different from qualia, “what it is like” to perceive an object? The distinction implies that representational content is a component of perceptual experience that is not experienced in any particular way, which is absurd – “what it is like” for one to have a perceptual experience is what it is like to have its object represented to him. How else could representational content possibly function? There is no way that content could be represented to the perceiver except by his experiencing it, nor is there anything outside of his experience that he is aware of when he perceives something. The nature of representational content thus neutralizes the need for qualia to explain experience.
Huemer’s explanation of the difference between red and red* (the quale) is based on a confusion. He states that “…red* is a property of experience, whereas red … is a property of physical objects”. It is correct to separate these as properties of two different types, but the way in which he speaks of them is misleading. Red as a property of physical objects is not a color, but rather the tendency to reflect certain wavelengths of light. Color, on the other hand, is an experienced property. It would be more logical for red* to refer to the physical property of reflecting light and for red to indicate the color that is experienced. From this perspective, it is unnecessary to posit the existence of qualia because we no longer have the sense that the color we experience is something strange and illegitimate that must be explained away.
Upon asking whether there is any reason for qualia to exist or if they serve any purpose, Huemer exhibits more unintentional humor by responding “I do not know the answer to this”, yet still insisting that they must exist. He offers two potential answers, neither of which is very convincing. First, he claims that the information obtained through perceptual experiences could not be represented without qualia. The very fact that he uses the word “represented” betrays his argument. He asks how we could perceive red without qualia and swiftly concludes that it is impossible, but it is perfectly plausible that we perceive red by way of representational content, which represents the object as being red, and therefore have no reason to posit qualia. Second, he asserts that qualia serve a conative function, expressing the pain or pleasure of an experience. There is no obvious reason why representational content should not perform this function as well, representing a warm experience as pleasant or a red experience as striking. He argues that qualia “simultaneously give us information about the world and give us emotional reactions or desires. Those two functions are integrated into the same experience, rather than being functions of two separate states or events”. It is the purpose of representational content to give us information about the world; if the two functions are integrated, it makes more sense to simply assign them to representational content and discard qualia as redundant.

Objection 4: Sense Data and Representationalism

When he explains how his theory is a form of direct realism, Huemer makes a dangerous statement: “We might also be said to be aware (directly and primarily) of the colors and shapes of the (facing surfaces of) physical objects around us, since that could also be described as what satisfies the content of a state representing there to be objects of those colors and shapes”. Although he describes this view as if it were only a trivial variation of his previous claim, it opens the door to a host of opposing theories that he wants to avoid. In particular, it allows for the possibility of a sense datum theory, which in turn implies representationalism.
He ends this section by saying “We must therefore conclude that direct realism is true”. This conclusion follows from his original claim that we are directly aware of the existence of objects, but it does not follow from the sense datum theory he implies in his second claim. According to the latter theory, we are directly aware of sense data, not objects. In order to have awareness of objects, one must undergo a non-perceptual process of judgment, forming a theory that infers the existence of objects from the sense data. Therefore, sense data do “satisfy the content of a state representing there to be objects”, but they do not do so directly because the state is based on the sense data. By admitting the plausibility of this view, Huemer is in danger of committing himself to indirect realism.
The axe and wood analogy Huemer provides is clever and useful, but it does not grant him immunity from sense datum theory either. He intends the physical objects to be the “wood” and perceptual experience the “axe”, but the analogy works equally well if we replace “physical objects” with “sense data.” The argument would go as follows: We use perceptual experience as a tool that enables us to perceive sense data directly. As a consequence, we are aware of physical objects only indirectly. Huemer could object that sense data are also vehicles of perception, but his analogy betrays him again. If perceptual experience is the vehicle of perception, sense data must be the objects of perception. Once the sense data have been perceived, the analogy repeats: sense data become the vehicle by which we become aware of physical objects. At this point, however, awareness has already become indirect. In order to use them as means, one must first obtain the sense data as objects. When used as means, they are not vehicles of perception but rather of judgment – all the necessary perception has already occurred. Physical facts, in turn, become the objects of judgment and are thus distanced from the direct relationship with perception that Huemer wants to maintain.

Conclusion

For the most part, Huemer’s theory is plausible and supports his theses. Objections one through three do have some validity but are not strong enough to falsify his system in general. Some can be defended against, while others can be reconciled to his account with only minor revisions. Of the first three, objection three is the most radical, but Huemer could address it easily. Neither the existence of qualia nor the independence of forcefulness is so central to his argument that his theory would be irreparably damaged by removing them, and all the phenomena they explain could be preserved in the updated account.
Objection four, if correct, deals a heavy blow to Huemer’s original theses. Sense datum theory would rewrite his first thesis to say that perception is direct awareness of sense data. The second thesis would then have to be revised to say that we have indirect, inferential knowledge of the external world as a result of the sense data gathered by perception. Although Huemer only becomes explicitly vulnerable to sense datum theory at the end of the chapter, the rest of his account would not need extensive alteration to accommodate sense data.
If we replace his references to physical objects with references to sense data, we arrive at the following theory: perception consists of an internal mental state, an object that produces sense data in the perceiver, and a causal relationship between the mental state and the sense data. The internal mental state consists of the experiences of perceiving sense data (whether or not these are called qualia), whose contents represent patches of color as immediately present (whether or not this is called forcefulness). Therefore, perception is direct awareness of sense data, which leads to indirect knowledge of the external world by a process of judgment.
This is by far the greatest threat to Huemer’s project – it challenges his central theses without contradicting the majority of his theory, making it difficult for him to argue against. One defense Huemer could mount is that we are not aware of any process of judgment between perception and awareness. If sense data require judgment to be seen as objects, why do we see objects in the world without having to reflect on them first? The task of filling the breaches in this theory, however, lies with Huemer himself.

REFERENCE
Huemer, Michael. Skepticism and the Veil of Perception.Oxford: Rowman and Littlefield Publishers, Inc., 2001.

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