by Thomas Raleigh
Here are 2 very natural ideas about conscious experiences:
(1) They are in some deep, interesting sense private.
(2) Nevertheless, different subjects can have qualitatively similar experiences.
I think that these 2 ideas are in tension. If we take seriously the idea that it is metaphysically impossible for any other subject to experience my phenomenal experiences, then the notion of phenomenal similarity across different subjects will not be well defined. And the reason for this is that the very idea of a phenomenal or appearance property – how something looks, feels or appears – is an inherently relativistic notion. That is: how something looks/feels/appears is always relative to some kind of subjective viewpoint or sense modality.
What is the relevant sense of ‘private’? Roughly: it is metaphysically impossible for anyone else to ‘directly’ experience my token phenomenal states/property instances. (Slightly less roughly: it is impossible for the token phenomenal features which constitutively form my stream of consciousness, to likewise constitutively figure in anyone else’s stream of consciousness.)
A traditional sort of scepticism raises the worry that we cannot ever know whether our respective private features are qualitatively the same – maybe your experience when looking at a ripe tomato is privately very different from mine; maybe your sensations when you bang your funny bone are quite different to how banging my funny bone feels to me; maybe, despite your fluent talk about experiences of various kinds, you actually have no inner conscious life whatsoever; and so on.
However, even allowing that this sort of sceptic may have a point, the following thought can seem very plausible: ok, perhaps I cannot be sure that your private stuff is the same as mine. But so long as it is in fact the same (and we both suppose or believe this to be so even if we may not be able to know it), then we can at least still successfully communicate and understand each other’s claims about our respective private stuff.
But notice that this line of thought relies on the second idea, above: that there can be facts, even if they may be unknowable, concerning inter-subjective phenomenal similarity. And this has been denied, at one time or other, by quite a few distinguished philosophers – including Frege, Schlick, Shoemaker and Stalnaker.
What is the problem then with the idea that 2 private phenomenal items, from 2 different subjects, could just naturally be similar – irrespective of what either subject can know or judge, or their dispositions to classify them and so on?
The crucial point here is that appearance properties and phenomenal properties are always relative to some specific (type of) subjective viewpoint. Nothing ever looks, feels or appears a certain way simpliciter; any item can only have some specific appearance/feel relative to some particular (type of) subjective viewpoint. The petals of this flower might look one way to humans, a different way to insects, yet another way to pigeons etc. But they don’t look any particular way at all simpliciter, or from no particular point of view. Nor, of course, do they look any way at all to a subject who entirely lacks vision – i.e. to a subjective viewpoint or modality that cannot ‘take in’ this item/feature. Likewise, my painful sensation feels a certain way to me, my after-image looks a certain way to me; but these token phenomenal features do not feel/appear any way at all to you, as your conscious point of view simply does not include them.
Given this essential relativity of appearance/phenomenal properties to a subjective perspective or sense-modality, two items can only be similar in appearance or feel relative to some particular subjective perspective or perspectives. Again: these two flowers may look similar to each other to humans, though they look very different from each other to pigeons. Or: they might both look identical (to me), yet they might each smell very differently (to me). But a claim that two items or features look or feel the same way relative to no particular conscious perspective or modality would be just as ill-defined as a claim that a single item has some specific appearance or feel relative to no particular conscious perspective or modality. And when it comes to two phenomenal items or features from two different private domains, there is, by hypothesis, no possibility that both items could ever ‘show up’ within a single subjective point of view – i.e. within a single subject’s conscious awareness. Thus it seems that any claim that my private item and your private item appear/feel phenomenally similar will not be a well-defined phenomenal comparison, for there is no possible common appearance space that both items could appear in.
In being essentially relative to a conscious perspective or modality, appearance/phenomenal properties are thus analogous to spatial and temporal properties – a point that I understand Wittgenstein to be making in section §350 of the Investigations:
It is as if I were to say, ‘You surely know what “It’s 5 o’clock here” means; so you also know what “It’s 5 o’clock on the sun” means. It means simply that it is just the same time there as it is here when it is 5 o’clock.’ – The explanation by means of sameness does not work here. For I know well enough that one can call 5 o’clock here and 5 o’clock there ‘the same time’, but I do not know in what cases one is to speak of its being the same time here and there. In exactly the same way, it is no explanation to say: the supposition that he has a pain is simply the supposition that he has the same as I.
Jennan Ismael makes a comparison with the notion of simultaneity in relativistic physics: there can be facts about temporal relations between 2 events within a spatial frame of reference. But when the 2 events are specified as belonging to different frames of reference, there is no fact as to one being earlier/later than the other. Likewise, given a model of experience which satisfies (MP), sameness or difference in the phenomenal appearance/feel of two private items/features may be well-defined relative to a particular subject’s private domain. But if the experiential elements in question belong to two different metaphysically private viewpoints, then any comparison of phenomenal appearance will not be well-defined. Just as there is no such thing as simultaneity simpliciter, but only relative to a specific frame of reference, there would be no such thing as phenomenal similarity simpliciter, but only within a specific private viewpoint.
One more comparison: John Divers has recently raised the following problem for Lewisian realism about possible worlds. If the truth of our modal claims is supposed to be explained by the existence of counterparts in distinct possible worlds, and those possible worlds are not spatio-temporally related to each other (i.e. spatio-temporal relations only obtain within individual possible worlds), then we face a problem with counter-factual claims like the following:
‘Usain might have been taller than he actually is.’
For the truth of such a claim is supposed to require the existence of a counterpart of Usain in some other possible world who is ‘taller than’ the actual Usain. But Usain in the actual world and Usain’s counterpart in some other possible world do not stand in any spatial relation; a fortiori they do not stand in the ‘taller than’ relation. In the absence of any possibility of actual-Usain and counterpart-Usain having their heights defined relative to a common spatio-temporal framework, we cannot make sense of claims that their respective heights are ‘the same’. Not only is it impossible for actual-Usain and counterpart-Usain to inhabit each other’s spatio-temporal domains, it is also impossible for any other item to function as a benchmark or ruler by which to meaningfully compare their heights – as it is metaphysically impossible for any item to figure in more than one Lewisian possible world.
Likewise, I am suggesting, assuming the metaphysical impossibility of my experiencing your private phenomenal items, or vice-versa, there is a lack of any common ‘appearance-space’ in which two items from distinct domains could have their subjective feels/appearances meaningfully compared as being phenomenally similar or different. Not only is it impossible for either of the two items in question to appear in the other’s private domain, it is also impossible for any other possible item/feature to serve as a phenomenal benchmark or standard in common to both private domains – as it is impossible for any item/feature to appear in more than one private phenomenal domain.
That is just the briefest sketch of the issue. But so what to do? I suggest that in order to hold onto the idea that we can have naturally similar phenomenal experiences, we should give up on the idea that they are metaphysically private – an idea that is in any case in tension with the physicalist idea that phenomenal experiences are ultimately just physical events/processes in the brain. There is nothing metaphysically private about the physical events and processes in a brain, which can be studied using 3rd-personal scientific methods just like other physical stuff. Indeed it seems plausible that it is precisely this supposedly special and unique conscious access we each have to our own sensations, mental imagery, innermost thoughts, etc. that provides one of the main motivations for dualism. As John Wisdom once remarked: “The peculiarity of the soul is not that it is visible to none but that it is visible to one”.
Taking seriously the idea that phenomenal similarity is always relative to a subjective viewpoint or appearance space – just as simultaneity and shape are relative to an observer’s frame of reference – thus gives us a reason to reject the perennially attractive idea that our conscious experiences are metaphysically private. But of course, others might want to jump the other way and hold onto phenomenal privacy at the expense of inter-subjective phenomenal similarity. The worry then is that this would make our privately defined phenomenal concepts incommunicable – but that will have to be a topic for another occasion.
If you are interested in the issues raised in this post, look out for Thomas' forthcoming article 'Phenomenal Privacy, Similarity and Communicability', which will be published soon in Ergo.