, I attempt to derive (what may be described as) the standard causal criterion for vision [see (3)*** below] from a critical examination of a non-causal definition of vision. By so doing I hope to make explicit some of the motivating intuitions that lie behind the causal account. The non-causal definition of vision with which I will like to begin may be schematically expressed as follows:
S sees an object O before him only if
(1) S has an experience E such that it looks just as if O is before him
(2) There is an object O before S
However, this definition hardly seems sufficient for saying that S sees O before him. For example, we may construct something akin to a Gettier-type counterexample to (1) and (2). (I say that the following example is merely ‘akin’ to a Gettier case because unlike the genuine Gettier counter-example mine is not concerned with the question of justification.) Imagine S has his eyes closed and is hallucinating that there is an object O before him. Suppose further that there actually is an object O before him. In such a case, the requirements of both (1) and (2) have been met, and yet we would not wish to say that S sees O before him since S would continue to have the experience E even if O was not present. This observation has motivated a causal account of perception that stipulates that S only counts as seeing O if E is caused by O. Thus we arrive at:
(3) S’s experience E is caused by O.
As it stands, the stipulation that E is caused by O does not seem sufficient since we can imagine instances when such causal dependence holds but in which we would still not count S as seeing O. Grice presents us with two such scenarios [Grice 1961, 141-5]. The first case is where S has his eyes closed and another person, P, describes the object O to S, which then prompts S to have a hallucination of O. In such a case, S’s experience E is caused by O, albeit indirectly via P’s description. Since E is ultimately causally dependent on O, the demands of (3) have been met. Yet, we would still be unwilling to say that in such a scenario S sees O. Thus, as it stands, the addition of (3) is still not sufficient for defining a case of vision.
One possible reply to Grice’s first example would be to point out that it is highly unlikely that P’s description of O could give rise to a hallucination in S that perfectly matched O. Thus, if we added a clause to (3) requiring that E perfectly match O we would (it is supposed) exclude cases of hallucinations based on the description of an object. Thus we arrive at something like:
(3)* S’s experience E is caused by O and E perfectly matches O.
At least three points can be made with regards to the above reply to Grice’s first example. First, it is not clear that it is impossible for P to describe O in enough detail for S to have a hallucination of O that fulfilled the requirement of (3)*. Admittedly, this may be difficult to do depending on how complex O happens to be, but it certainly isn’t (strictly speaking) impossible. In fact, it may even turn out that the perfect match between E and O may simply be a matter of luck, this would still be sufficient to fulfill the requirement of (3)*, so long as E is (indirectly) caused by O. Moreover, we could imagine a case in which O is a simple geometric shape such as a triangle or square. In such a case it wouldn’t seem all that difficult for P to describe O in such a way that S’s hallucination of O perfectly matched O.
Second, (3)* does not seem like a plausible necessary condition for vision since there are many cases in which we may be willing to say that S sees O although E does not perfectly match O. For example, let us suppose that S sees a white shaggy dog, which he mistakes for a sheep. In such a case, S’s experience E may not perfectly match O and yet it is not clear that we would wish to deny that S sees O. Other examples include misreading the letters (or words) on a street sign and certain cases of blurred vision. Moreover, there is at least one reading of ‘perfectly matches’ in which it wouldn’t make sense to say any experience E perfectly matches (or in any way matches) O. On such a reading experiences may be taken to be a very different kind of thing from the objects that give rise to them. While I’m not particularly keen on pushing for such an interpretation of experience vis-à-vis objects, it does seem to be a viable position one may hold.
Our third and final objection to (3)* comes from Grice’s second counterexample. Grice imagines a scientist who looks at an object O and stimulates S’s brain in exactly the way necessary for S to have an experience E that matches O. If instead of a scientist we substituted a sophisticated computer, then the possibility of E perfectly matching O perfectly (at least as perfect as we could reasonably expect in a normal case of vision) becomes quite plausible. Given this fact, along with the fact that E remains causally dependent on O, the demands of (3)* have once again been met. Nevertheless, we would still be unwilling to say that in such a scenario S sees O. Thus, the addition of (3)* still leaves us with an insufficient causal criterion for vision.
Another way one may object to Grice’s examples is by pointing out that in both the causal dependence of E on O is indirect. If we stipulate that the causal dependence must be direct we arrive at:
(3)** S’s experience E is caused by O directly.
The word ‘directly’, as used in (3)** suggests that there cannot be any mediating factors between O and E. However, this does not seem like a plausible requirement for vision since there are cases where O may cause E indirectly where we would nevertheless still wish to insist that S sees O. For example, imagine that S is looking at bacteria on a slide with the aid of a powerful microscope. In such a case, E’s causal dependence on O is mediated (i.e., by the microscope) and yet we may still wish to insist that S sees O.
Although the requirement that O directly cause E seems problematic, it does hint at a valuable intuition: namely, that there are a number of possible causal scenarios linking E to O that would be unacceptable as far as our putative understanding of vision is concerned. According to this line of reasoning our concept of vision includes the notion that there should be a certain type of causal relationship between perceptual experiences and their objects. Thus, for S to see O, his perceptual experiences must be caused in the right sort of way, where this is understood as excluding all causal chains that deviate from ‘the norm’. What constitutes ‘the norm’ can be arrived at easily enough by examining unproblematic instances (i.e., paradigm cases) where the locution ‘S sees O’ correctly applies. Examples, such as Grice’s discussed above, represent causal chains that deviate from the norm and therefore fail to be causally relevant in the right way. If we add the requirement excluding deviant causal chains to (3) we arrive at:
(3)*** S’s experience E is caused by O in the right sort of way.
Admittedly, what the expression ‘right sort of way’ entails needs to be spelt on in greater detail. Nevertheless, (3)*** is regarded by many to reflect the canonical statement of the causal requirement of vision and is taken, along with (1) and (2) to be sufficient for establishing that S sees O.
The causal criterion we have derived represents merely the first step, rather than the end, of the debate regarding whether the causal description of vision is the best available. For example, many believe disjunctivism represents an attractive alternative to the causal picture, though it remains unclear whether the two approaches are necessarily incompatible. Moreover, while (1), (2) and (3)*** are here taken to be sufficient for vision, another question worth considering is whether they also represent a necessary condition.