There are two pre-theoretic claims made by ordinary people who have not studied philosophy; absent that event, most people tend to be direct realists. The two claims are as follows.
(A): The direct objects of perception are typically mind-independent external objects.
(B): Perception involves a direct relationship to its objects.
Few philosophers defend direct realism. So they will deny one of the claims. Claim (B) is denied by intentional theories. Claim (A) is denied by sense data theories.
This essay will outline the arguments in general for sense data or equivalently against (A), and then consider in detail the related positions of Valberg and Snowdon.
2. General Arguments For Sense Data
2.1. Argument From Commonality of Stimuli
Many observers deny claim (A) without denying the sub-claim that mind-independent external objects exist. There seem to be commonalities in our perceptions of the same object at different times and for different observers. While we cannot rule out possibilities such as inverted spectra for different observers, we can observe that everyone leaves a room by the door that therefore appears to be perceived as having the same location for all observers.
The inverted spectrum possibility does however raise the prospect of a mismatch between what different observers perceive in detail: I may see an object as red with an experience that would be described as green by another observer. This would not be detectable in behavior because we would both have acculturated.
If two people can perceive the same object differently, then they must directly perceive sense data to allow for the difference to exist. This may be objected to in a transcendentally idealistic vein by claiming that in fact our sensory modalities create much of what an object appears to be is created by the observer in any case. However there would still be scope for different observers to ‘create’ different objects and this difference must be accommodated somewhere.
2.2. Argument From Illusion/Hallucination
It is possible for people to perceive objects incorrectly to have properties they do not have or indeed to perceive objects that are in fact not there at all. Examples of the former would include looking at a white wall under a red light that is not itself seen. Subjects may incorrectly believe that the wall is red. Examples of the latter include afterimages that are caused by looking at a bright light source. These persist after the subject is no longer looking at the source and thus the perceived object is not related to a currently existing physical object at that location.
In both of these cases, the information that the perception is false – i.e. does not relate to an actual physical object present at that location at that time with those properties perceived – will typically not be available to the subject purely through perceptual channels. They may be able to reason from prior experience of afterimages and the unlikelihood of a reddish circular diaphanous object appearing soon after accidentally viewing the sun to the truth of the matter. But perceptually all that will be available is the illusory percepts. It is therefore held that there will be no relevant difference in the perceiver in the case of a veridical perception of an external object and a hallucinatory perception of a non-present external object.
Subjects must have internal representations of their percepts in the case of non-veridical perceptions because there is no external object to allow for the supply of perceptual data from outside. Since there is no difference accessible to the subject in veridical cases and illusory ones, these internal representations – or sense data – must also operate in normal cases and in fact all cases of perception. Direct realists and any others who wish to retain claim (A) must accept that different types of situation are occurring in veridical and hallucinatory perception; this is perhaps unpalatable and I would not endorse it but it is certainly not ruled out by the absence of differential phenomenology.
As this experimental field develops, few things are clearer than that the brain is immensely complex and has an extremely large number of processes; visual processing drives a major part of this complexity. This is taken to be an argument against claim (A) in that these manifold processes must be having significant effects on the raw input – this is in some ways another transcendental idealism line. Dancy dismisses this by asking in what way these processes are intermediaries between the perceiver and the object. However his main argument for this is lack of relevant phenomenology. This seems inadequate because I also have no phenomenology in respect of photons as such travelling to me from distant stars and those must be intermediaries. In sum, it seems improbable that immense amounts of processing in the primary visual cortex V1, containing 140m neurons though only one of five visual areas, leaves the input untouched.
2.4. Time Lag Argument
The speed of light is finite and the light from distant objects may take appreciable time to arrive at the retina of a perceiver. In the case of very distant objects such as stars, this time lag may be a thousand years. The star itself may have ceased to exist five hundred years previously to the light from it being perceived. Thus, claim (A) must be false because no external object corresponding to the perception exists at the time of the perception.
This argument is then extended to all objects of perception by observing that given a finite speed of light, there will always be some lag for all objects at a non-zero displacement to the perceiver.
This argument seems weak because it may be objected that claim (A) does not mention time lag effects. There cannot now be a direct relationship with an object that is no longer present; but there can be one with an object that was present when it emitted the light. Or perhaps the direct relationship is with the light that is in fact now present. Though the counter to this counter is to note that claim (A) was not supposed to be arguing for a direct relationship to light but to external objects.
Valberg’s paper falls into two parts; he discusses an ‘antinomy’ between reasoning about visual experience and the relevant phenomenology. The reasoning is essentially that discussed above in favor of sense data while the opposition is that if there are in fact sense data, they must be transparent because we do not experience them: all we experience is the world. This line is somewhat redolent of Hume’s discussion of finding skeptical arguments convincing until he returns to company and backgammon; and Valberg’s conclusion has a Wittgensteinian air: ordinary life is the cure for philosophy.
3.1. Arguments For Sense Data
Valberg’s main line involves defining ‘demonstrative reference’, by which he means the ability to speak of that book which is an object of his experience. He deliberately leaves open the ambiguity initially as to whether that book as an object of experiences is external or internal because this is precisely the point at issue. Whichever way that question is resolved, it still makes sense to talk in this way.
Valberg then runs a detailed version of the Argument From Hallucination, noting that it is logically possible for an external object to be eliminated and yet the brain activity of the perceiver remain unaltered. Therefore “it would in some sense be true that ‘how things are in my experience would remain the same’ ”. Note that Valberg continues to leave open the key question: he wishes to restrict ‘experience’ to the brain activity. If experience of the book extended to the external object, he would have begged the question.
The final stage of what Valberg terms the ‘problematic reasoning’ uses the demonstrative reference previously introduced. He now uses the term this to refer to an object present to him in his experience. He is “focused on whatever it is that is present in his experience while I am looking at the book”. He notes that it is possible for him to hold his attention focused in this way and yet also possible that the book was eliminated say half way through a five second period without his experience changing. Despite the unchanging experience, there has been a significant rupture in the world with a book popping out of existence. So this object is not the book in the external world and claim (A) is false. While this argument seems strong, it is not clear what Valberg has added to the canonical formulations beyond a certain time slicing.
3.2. Openness To Experience
The second half of the antinomy results from the conflict between the problematic reasoning and the fact that in experience, there is only one object present. The book is this object and it is on the table and not in the mind. So indirectly, argument suggests that this is sense data while, directly, being open to our experience suggests the opposite.
Valberg then takes a phenomenological/existentialist line. Sartre’s concept of bad faith is mentioned; this is to refer to the fact that we are not really surprised when we see a book in the world as an external object. This is because while we in some way know or accept the problematic reasoning, we do not or are unable to follow such beliefs: our dispositions to behave will inevitably be based on the idea that the book is out there to be read and handled. But the possibility of bad faith arises only for people who have considered the idea of sense data because only those persons can lie to themselves. Everyone else is simply engaged with the world. This talk of engagement and handling naturally sets the scene for the introduction of Heidegger, who is cited paradoxically referring to the need to “first leap onto the soil on which we really stand”.
The key point is that we have to ask the question as to what mechanism will allow us to adjudicate between competing frameworks for judgement. There seems to be no actual arena in which the two halves meet; worse, both are supreme in their own spheres. Surely we cannot use reasoning to judge whether the problematic reasoning is correct: on the basis of reason it surely is because experience does not enter the arena. However, if we care about how it seems to us, and more, importantly how it appears inevitably and unchangeably to appear to us, we obtain the opposite result. Can we on the other hand use experience to privilege experience over reasoning? Not without solving this particular version of a problem of induction. Valberg concludes on the rather aporetic note that he has failed to solve the conflict and also does not believe it can be dismissed as illegitimate.
Snowdon wishes to examine the term ‘direct perception’ which he abbreviates by considering a subject who d-perceives an object. He introduces two constraints on his theory. Firstly, it should account for the fact that everyone pre-theoretically supports claim (A). Secondly, since the denial of claim (A) amounts to a claim that there is no direct relationship to external objects, the theory must make this possible. So there must be sufficient conceptual space to allow for the truth of the relationship claim because otherwise it has effectively been stipulated as false rather than investigated. So x d-perceives y should be an extensional two-place relation and the question is whether x can be an observer and y an external object.
Snowdon first considers epistemological interpretations of d-perception, citing Russell’s claim that the table must be an inference from what is immediately known. This cannot mean that we can see different things when we use inference than otherwise. The difference is that I can know immediately that something is green but I must then additionally infer that it is an apple.
Snowdon then dismisses these epistemological interpretations on two bases. First, they fail to satisfy the second constraint listed above in that they produce a relationship between the perceiver and some facts and not the perceiver and some objects. Second, it produces an intensional context because the question (X) ‘do I d-perceive that thing?’ has no unique answer; the answer will change depending on what other facts and relationships the observer commands. An intensional context is an unhelpful element in a theory of d-perception because these related but separate facts will alter the truth conditions of the question (X).
The alternative non-epistemological interpretation of d-perceives introduced by Snowdon relies on a similar demonstrative concept to the one discussed by Valberg: Snowdon claims that x d-perceives y if the relationship between x and y is such that x could make the true demonstrative judgment that ‘that is y’. Sense data theories will then go on to deny this via claim (A) that holds that y can only be filled by internal objects.
This rather subtly fulfills the first constraint because it is effective whether claim (A) is correct or false. If it is false, people are making correct judgments about sense data; otherwise they are making correct judgements about external objects. It also fulfills the second constraint because it is a two-place extensional relation.
The objection that this would deny perception to non-cognitively adept entities such as non-human animals is met by pointing out that the d-perception description merely describes the subject-object relation that would allow for true demonstrative thoughts when sufficient cognitive ability existed.
Snowdon’s picture has up to this point deferred the question on claim (A) and is consistent with its falsity and its truth. He now considers Hume’s version of the Argument From Illusion, which relates to a table apparently diminishing as we move. Clearly that table has properties not possessed by any mind-independent table and so they are not identical. Snowdon also covers the time lag argument by holding that the finite speed of light means it is possible for us to demonstratively think about that star even though it may no longer exist. The weight of his position is predominantly for claim (A).
These arguments are outlined in Dancy, Introduction to Contemporary Epistemology, Blackwell Publishing, p. 152
G Leuba, R Kraftsik, Changes in volume, surface estimate, three-dimensional shape and total number of neurons of the human primary visual cortex from midgestation until old age, Anat Embryol (1994) 190:351-366
J Valberg, The puzzle of experience, in T Crane (ed), The contents of experience, Cambridge University Press, 1992
Ibid, p. 42, Valberg refers to M Heidegger, What is Called Thinking, Harper and Row, 1968, p. 41
P Snowdon, How to interpret ‘direct perception’, in T Crane (ed), The contents of experience, Cambridge University Press, 1992