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A final position that should be mentioned is the state theory, according to which a sense experience of an ‘A’ is an occurrent, non-relational state of the kind that the subject would be in when perceiving an ‘A’. Suitably qualified, this claim is no doubt true, but its significance is subject to debate. Here it is enough to remark that the claim is compatible with both pure cognitivism and the adverbial theory, and that state theorists are probably best advised to adopt adverbials as a means of developing their intuitions.


Yet, clarifying sense-data, if taken literally, is that which is given by the senses. But in response to the question of what exactly is so given, sense-data theories posit private showings in the consciousness of the subject. In the case of vision this would be a kind of inner picture show which itself only indirectly represents aspects of the external world that has in and of itself a worldly representation. The view has been widely rejected as implying that we really only see extremely thin coloured pictures interposed between our mind’s eye and reality. Modern approaches to perception tend to reject any conception of the eye as a camera or lense, simply responsible for producing private images, and stress the active life of the subject in and of the world, as the determinant of experience.

Nevertheless, the argument from illusion is of itself the usually intended directive to establish that certain familiar facts about illusion disprove the theory of perception called naïevity or direct realism. There are, however, many different versions of the argument that must be distinguished carefully. Some of these distinctions centre on the content of the premises (the nature of the appeal to illusion); others centre on the interpretation of the conclusion (the kind of direct realism under attack). Let ‘us’ set about by distinguishing the importantly different versions of direct realism which one might take to be vulnerable to familiar facts about the possibility of perceptual illusion.

A crude statement of direct realism might go as follows. In perception, we sometimes directly perceive physical objects and their properties, we do not always perceive physical objects by perceiving something ‘else’, e.g., a sense-datum. There are, however, difficulties with this formulation of the view, as for one thing a great many philosophers who are ‘not’ direct realists would admit that it is a mistake to describe people as actually ‘perceiving’ something other than a physical object. In particular, such philosophers might admit, we should never say that we perceive sense-data. To talk that way would be to suppose that we should model our understanding of our relationship to sense-data on our understanding of the ordinary use of perceptual verbs as they describe our relation to and of the physical world, and that is the last thing paradigm sense-datum theorists should want. At least, many of the philosophers who objected to direct realism would prefer to express in what they were of objecting too in terms of a technical (and philosophically controversial) concept such as ‘acquaintance’. Using such a notion, we could define direct realism this way: In ‘veridical’ experience we are directly acquainted with parts, e.g., surfaces, or constituents of physical objects. A less cautious verison of the view might drop the reference to veridical experience and claim simply that in all experience we are directly acquainted with parts or constituents of physical objects. The expressions ‘knowledge by acquaintance’ and ‘knowledge by description’, and the distinction they mark between knowing ‘things’ and knowing ‘about’ things, are generally associated with Bertrand Russell (1872-1970), that scientific philosophy required analysing many objects of belief as ‘logical constructions’ or ‘logical fictions’, and the programme of analysis that this inaugurated dominated the subsequent philosophy of logical atomism, and then of other philosophers, Russell’s "The Analysis of Mind," the mind itself is treated in a fashion reminiscent of Hume, as no more than the collection of neutral perceptions or sense-data that make up the flux of conscious experience, and that looked at another way that also was to make up the external world (neutral monism), but "An Inquiry into Meaning and Truth" (1940) represents a more empirical approach to the problem. Yet, philosophers have perennially investigated this and related distinctions using varying terminology.

Distinction in our ways of knowing things, highlighted by Russell and forming a central element in his philosophy after the discovery of the theory of ‘definite descriptions’. A thing is known by acquaintance when there is direct experience of it. It is known by description if it can only be described as a thing with such-and-such properties. In everyday parlance, I might know my spouse and children by acquaintance, but know someone as ‘the first person born at sea’ only by description. However, for a variety of reasons Russell shrinks the area of things that can be known by acquaintance until eventually only current experience, perhaps my own self, and certain universals or meanings qualify anything else is known only as the thing that has such-and-such qualities.

Because one can interpret the relation of acquaintance or awareness as one that is not ‘epistemic’, i.e., not a kind of propositional knowledge, it is important to distinguish the above aforementioned views read as ontological theses from a view one might call ‘epistemological direct realism? In perception we are, on at least some occasions, non-inferentially justified in believing a proposition asserting the existence of a physical object. Since it is that these objects exist independently of any mind that might perceive them, and so it thereby rules out all forms of idealism and phenomenalism, which hold that there are no such independently existing objects. Its being to ‘direct’ realism rules out those views defended under the cubic of ‘critical naive realism’, or ‘representational realism’, in which there is some non-physical intermediary - usually called a ‘sense-datum’ or a ‘sense impression’ -that must first be perceived or experienced in order to perceive the object that exists independently of this perception. Often the distinction between direct realism and other theories of perception is explained more fully in terms of what is ‘immediately’ perceived, than ‘mediately’ perceived. What relevance does illusion have for these two forms of direct realism?

The fundamental premise of the arguments is from illusion seems to be the theses that things can appear to be other than they are. Thus, for example, straight sticks when immerged in water looks bent, a penny when viewed from certain perspective appears as an illusory spatial elliptic circularity, when something that is yellow when place under red fluorescent light looks red. In all of these cases, one version of the argument goes, it is implausible to maintain that what we are directly acquainted with is the real nature of the object in question. Indeed, it is hard to see how we can be said to be aware of the really physical object at all. In the above illusions the things we were aware of actually were bent, elliptical and red, respectively. But, by hypothesis, the really physical objects lacked these properties. Thus, we were not aware of the substantial reality of been real as a  physical objects or theory.

So far, if the argument is relevant to any of the direct realisms distinguished above, it seems relevant only to the claim that in all sense experience we are directly acquainted with parts or constituents of physical objects. After all, even if in illusion we are not acquainted with physical objects, but their surfaces, or their constituents, why should we conclude anything about the hidden nature of our relations to the physical world in veridical experience?

We are supposed to discover the answer to this question by noticing the similarities between illusory experience and veridical experience and by reflecting on what makes illusion possible at all. Illusion can occur because the nature of the illusory experience is determined, not just by the nature of the object perceived, but also by other conditions, both external and internal as becoming of an inner or as the outer experience. But all of our sensations are subject to these causal influences and it would be gratuitous and arbitrary to select from indefinitely of many and subtly different perceptual experiences some special ones those that get ‘us’ in touch with the ‘real’ nature of the physical world and its surrounding surfaces. Red fluorescent light affects the way thing’s look, but so does sunlight. Water reflects light, but so does air. We have no unmediated access to the external world.

Still, why should we consider that we are aware of something other than a physical object in experience? Why should we not conclude that to be aware of a physical object is just to be appeared to by that object in a certain way? In its best-known form the adverbial theory of something proposes that the grammatical object of a statement attributing an experience to someone be analysed as an adverb. For example,

(A) Rod is experiencing a coloured square.

Is rewritten as?

Rod is experiencing, (coloured square)-ly

This is presented as an alternative to the act/object analysis, according to which the truth of a statement like (A) requires the existence of an object of experience corresponding to its grammatical object. A commitment to the explicit adverbializations of statements of experience is not, however, essential to adverbialism. The core of the theory consists, rather, in the denial of objects of experience (as opposed ti objects of perception) coupled with the view that the role of the grammatical object in a statement of experience is to characterize more fully te sort of experience that is being attributed to the subject. The claim, then, is that the grammatical object is functioning as a modifier and, in particular, as a modifier of a verb. If it as a special kind of adverb at the semantic level.

At this point, it might be profitable to move from considering the possibility of illusion to considering the possibility of hallucination. Instead of comparing paradigmatic veridical perception with illusion, let ‘us’ compare it with complete hallucination. For any experiences or sequence of experiences we take to be veridical, we can imagine qualitatively indistinguishable experiences occurring as part of a hallucination. For those who like their philosophical arguments spiced with a touch of science, we can imagine that our brains were surreptitiously removed in the night, and unbeknown to ‘us’ are being stimulated by a neurophysiologist so as to produce the very sensations that we would normally associate with a trip to the Grand Canyon. Currently permit ‘us’ into appealing of what we are aware of in this complete hallucination that is obvious that we are not awaken to the sparking awareness of physical objects, their surfaces, or their constituents. Nor can we even construe the experience as one of an object’s appearing to ‘us’ in a certain way. It is after all a complete hallucination and the objects we take to exist before ‘us’ are simply not there. But if we compare hallucinatory experience with the qualitatively indistinguishable veridical experiences, should we most conclude that it would be ‘special’ to suppose that in veridical experience we are aware of something radically different from what we are aware of in hallucinatory experience? Again, it might help to reflect on our belief that the immediate cause of hallucinatory experience and veridical experience might be the very same brain event, and

it is surely implausible to suppose that the effects of this same cause are radically different  -acquaintance with physical objects in the case of veridical experience: Something else in the case of hallucinatory experience.

This version of the argument from hallucination would seem to address straightforwardly the ontological versions of direct realism. The argument is supposed to convince ‘us’ that the ontological analysis of sensation in both veridical and hallucinatory experience should give ‘us’ the same results, but in the hallucinatory case there is no plausible physical object, constituent of a physical object, or surface of a physical object with which additional premiss we would also get an argument against epistemological direct realism. That premiss is that in a vivid hallucinatory experience we might have precisely the same justification for believing (falsely) what we do about the physical world as we do in the analogous, phenomenological indistinguishable, veridical experience. But our justification for believing that there is a table before ‘us’ in the course of a vivid hallucination of a table are surely not non-inferential in character. It certainly is not, if non-inferential justifications are supposedly a consist but yet an unproblematic access to the fact that makes true our belief  -by hypothesis the table does not exist. But if the justification that hallucinatory experiences give ‘us’ the same as the justification we get from the parallel veridical experience, then we should not describe a veridical experience as giving ‘us non-inferential justification for believing in the existence of physical objects. In both cases we should say that we believe what we do about the physical world on the basis of what we know directly about the character of our experience.

In this brief space, I can only sketch some of the objections that might be raised against arguments from illusion and hallucination. That being said, let us begin with a criticism that accepts most of the presuppositions of the arguments. Even if the possibility of hallucination establishes that in some experience we are not acquainted with constituents of physical objects, it is not clear that it establishes that we are never acquainted with a constituent of physical objects. Suppose, for example, that we decide that in both veridical and hallucinatory experience we are acquainted with sense-data. At least some philosophers have tried to identify physical objects with ‘bundles’ of actual and possible sense-data.

To establish inductively that sensations are signs of physical objects one would have to observe a correlation between the occurrence of certain sensations and the existence of certain physical objects. But to observe such a correlation in order to establish a connection, one would need independent access to physical objects and, by hypothesis, this one cannot have. If one further adopts the verificationist’s stance that the ability to comprehend is parasitic on the ability to confirm, one can easily be driven to Hume’s conclusion:


Let us chance our imagination to the heavens, or to the utmost limits of the universe, we never really advance a step beyond ourselves, nor can conceivable any kind of existence, but those perceptions, which have appear d in that narrow compass. This is the universe of the imagination, nor have we have any idea but what is there  Reduced. (Hume, 1739-40).

If one reaches such a conclusion but wants to maintain the intelligibility and verifiability of the assertion about the physical world, one can go either the idealistic or the phenomenalistic route.

However, hallucinatory experiences on this view is non-veridical precisely because the sense-data one is acquainted with in hallucination do not bear the appropriate relations to other actual and possible sense-data. But if such a view were plausible one could agree that one is acquainted with the same kind of a thing in veridical and non-veridical experience but insists that there is still a sense in which in veridical experience one is acquainted with constituents of a physical object?

A different sort of objection to the argument from illusion or hallucination concerns its use in drawing conclusions we have not stressed in the above discourses. In mentioning this objection, it may perhaps underscore an important feature of the argument. At least some philosophers (Hume, for example) have stressed the rejection of direct realism on the road to an argument for general scepticism with respect to the physical world. Once one abandons epistemological; direct realisms, one has an uphill battle indicating how one can legitimately make the inferences from sensation to physical objects. But philosophers who appeal to the existence of illusion and hallucination to develop an argument for scepticism can be accused of having an epistemically self-defeating argument. One could justifiably infer sceptical conclusions from the existence of illusion and hallucination only if one justifiably believed that such experiences exist, but if one is justified in believing that illusion exists, one must be justified in believing at least, some facts about the physical world (for example, that straight sticks look bent in water). The key point to stress in relying to such arguments is, that strictly speaking, the philosophers in question need only appeal to the ‘possibility’ of a vivid illusion and hallucination. Although it would have been psychologically more difficult to come up with arguments from illusion and hallucination if we did not believe that we actually had such experiences, I take it that most philosophers would argue that the possibility of such experiences is enough to establish difficulties with direct realism. Indeed, if one looks carefully at the argument from hallucination discussed earlier, one sees that it nowhere makes any claims about actual cases of hallucinatory experience.

Another reply to the attack on epistemological direct realism focuses on the implausibility of claiming that there is any process of ‘inference’ wrapped up in our beliefs about the world and its surrounding surfaces. Even if it is possible to give a phenomenological description of the subjective character of sensation, it requires a special sort of skill that most people lack. Our perceptual beliefs about the physical world are surely direct, at least in the sense that they are unmediated by any sort of conscious inference from premisses describing something other than a physical object. The appropriate reply to this objection, however, is simply to acknowledge the relevant phenomenological fact and point out that from the perceptive of epistemologically direct realism, the philosopher is attacking a claim about the nature of our justification for believing propositions about the physical world. Such philosophers need carry out of any comment at all about the causal genesis of such beliefs.

As mentioned that proponents of the argument from illusion and hallucination have often intended it to establish the existence of sense-data, and many philosophers have attacked the so-called sense-datum inference presupposed in some statements of the argument. When the stick looked bent, the penny looked elliptical and the yellow object looked red, the sense-datum theorist wanted to infer that there was something bent, elliptical and red, respectively. But such an inference is surely suspect. Usually, we do not infer that because something appears to have a certain property, that affairs that affecting something that has that property. When in saying that Jones looks like a doctor, I surely would not want anyone to infer that there must actually be someone there who is a doctor. In assessing this objection, it will be important to distinguish different uses words like ‘appears’ and ‘looks’. At least, sometimes to say that something looks ‘F’ way and the sense-datum inference from an F ‘appearance’ in this sense to an actual ‘F’ would be hopeless. However, it also seems that we use the ‘appears’/’looks’ terminology to describe the phenomenological character of our experience and the inference might be more plausible when the terms are used this way. Still, it does seem that the arguments from illusion and hallucination will not by themselves constitute strong evidence for sense-datum theory. Even if one concludes that there is something common to both the hallucination of a red thing and a veridical visual experience of a red thing, one need not describe a common constituent as awarenesses of something red. The adverbial

theorist would prefer to construe the common experiential state as ‘being appeared too redly’, a technical description intended only to convey the idea that the state in question need not be analysed as relational in character. Those who opt for an adverbial theory of sensation need to make good the claim that their artificial adverbs can be given a sense that is not parasitic upon an understanding of the adjectives transformed into verbs. Still, other philosophers might try to reduce the common element in veridical and non-veridical experience to some kind of intentional state.  More like belief or judgement. The idea here is that the only thing common to the two experiences is the fact that in both I spontaneously take there to be present an object of a certain kind.

The selfsame objections can be started within the general framework presupposed by proponents of the arguments from illusion and hallucination. A great many contemporary philosophers, however, uncomfortable with the intelligibility of the concepts needed to make sense of the theories attacked even. Thus, at least, some who object to the argument from illusion do so not because they defend direct realism. Rather they think there is something confused about all this talk of direct awareness or acquaintance. Contemporary Externalists, for example, usually insist that we understand epistemic concepts by appeal: To nomologically connections. On such a view the closest thing to direct knowledge would probably be something by other beliefs. If we understand direct knowledge this way, it is not clar how the phenomena of illusion and hallucination would be relevant to claim that on, at least some occasions our judgements about the physical world are reliably produced by processes that do not take as their input beliefs about something else.

The expressions ‘knowledge by acquaintance’ and ‘knowledge by description’, and the distinction they mark between knowing ‘things’ and knowing ‘about’ things, are now generally associated with Bertrand Russell. However, John Grote and Hermann von Helmholtz had earlier and independently to mark the same distinction, and William James adopted Grote’s terminology in his investigation of the distinction. Philosophers have perennially investigated this and related distinctions using varying terminology. Grote introduced the distinction by noting that natural languages ‘distinguish between these two applications of the notion of knowledge, the one being of the Greek  í íáé, nosene, Kennen, connaître, the other being ‘wissen’, ‘savoir’ (Grote, 1865). On Grote’s account, the distinction is a natter of degree, and there are three sorts of dimensions of variability: Epistemic, causal and semantic.

We know things by experiencing them, and knowledge of acquaintance (Russell changed the preposition to ‘by’) is epistemically priori to and has a relatively higher degree of epistemic justification than knowledge about things. Indeed, sensation has ‘the one great value of trueness or freedom from mistake’ (1900, p. 206).