The first objection is rather obvious: ``You baldly assert that a stretch of consciousness cannot be reversed. But the irreversibility of consciousness is far from self-evident. I can certainly imagine a person coming into or passing out of consciousness -- which seems to make it a reversible process. Do we not also say that people can `change their minds'?"
The first problem with this objection is that it appeals to senses of `consciousness' other than the sense we are employing. As we said above, we are specifically considering the question of whether subjective awareness (P-consciousness) over some stretch of time can be reversed. When Jones admits that he has ``changed his mind," he isn't saying anything remotely like, ``I just experienced the beauty of a sunset backwards." And when we say that Jones received a blow that caused him to lose consciousness, and that presently he was once again alert, we are saying at most that Jones was at one point in such and such a state of P-consciousness, at a subsequent point not in a state of P-consciousness, and then thereafter once again in some state of P-consciousness.
As to whether a stretch of P-consciousness is irreversible, we concede this much: the proposition in question isn't self-evident for those who haven't contemplated its truth value. In order to begin to gain an appreciation for the plausibility of the claim that consciousness is irreversible, one has but to try to pull off the reversal in one's own case. We ask, accordingly, that you indulge in a bit of introspection; we ask that you tackle the challenge of reversing a stretch of your own mental life. In order to fix things a bit, let's suppose that you are quite powerfully affected by hearing Beethoven's Ninth; suppose, more specifically, that you feel energized, deep down, when listening to an initial segment of the choral part of the Ninth during the interval of time to . (If you've never had this experience, merely substitute something similar.) Now, remember your experience during this stretch, fix it in your mind, and proceed to reverse it -- live it backwards, so to speak.
What happened? Not much, we wager. Part of the problem seems to be that a stretch of conscious experience can be what might be called indivisible. Perhaps if you could divide your Beethoven experience into sub-chunks corresponding to to , to , and so on, you could -- thought-experimentally speaking -- reassemble them in reverse. But for many, if not most, stretches of P-consciousness such division isn't possible.
We should not be read as proposing divisibility as a quodlibet around which the entire issue revolves. In fact, we're inclined to think that reversing a stretch of P-consciousness is simply incoherent. The challenge under consideration seems to us to be analogous to the challenge of imagining that 2+2=7, or that modus ponens is false. Such things can't be imagined -- because they are incoherent (to use a philosophical concept, they are logically impossible).
Now to say that a reversed experience of the Ninth is logically impossible is of course to make an exceedingly strong claim. For our main argument it suffices that such a reversed experience be physically impossible -- a more circumspect claim, and one we as a matter of fact retreat to below. However, after meditating on challenges like the one just posed, and on some states of affairs which are agreed to be logically impossible, it really does seem to us (and to those ``subjects" who, at our request, have genuinely attempted to conceive of some reversed stretches of P-consciousness) that the kind of reversal being called for in our challenge is logically impossible. We have no outright proof that P-consciousness is irreversible in this strong modal sense. Our rationale involves the observation that there are states of affairs deemed by all to be logically impossible (e.g., there being a book both 150 and 200 pages in length at the same time) not because they entail some violation of a truth in logic or mathematics, but rather because when one ponders them, nothing can be envisaged. The same ``nothingness" seems to attach to experiencing the Ninth in reverse. And Beethoven's Ninth isn't unique: other more mundane stretches of P-consciousness seem to be just as resistant to reversal: experiences of a dish of fresh strawberries and cream, a vigorous ski run, the reading of a short story, etc. -- all these scenarios seem to share the ``nothingness" of the 150/200 page book. It is easy enough to imagine swimming the Atlantic unaided, being twenty feet tall, enjoying immortality, finding a counter-example to Boyle's Law, and so on. But it's a tad harder to imagine that the circle has been squared -- about as hard, it seems to us, as imagining phenomenal awareness in reverse.