Suppose that at each time at which Jim experiences a pain, Sally experiences a pain that is exactly alike phenomenally, and vice versa. Suppose also that their attitudes to this kind of pain are the same. Then with respect to pain, neither is better off than the other.
But now let's add that Jim and Sally are born in the year 2000 and both die in 2040. But Jim's life proceeds twice as fast as Sally's (due to drugs or Special Relativity), so that in one minute of external time, Jim experiences two minutes of subjective time. Let's suppose that Jim and Sally have the same attitudes towards pain, and that Jim has a constant headache from his 20th subjective year until her 30th subjective year, while Sally has the same intensity of headache from her 10th subjective year until her 15th subjective year. Then Jim suffers that headache from 2010 through 2015, while Sally suffers it from 2010 until 2015. The headache is constant, so at every time at which Jim suffers a headache, Sally suffers exactly the same headache, and vice versa. And let's suppose that's all the pain either of them suffers.
By the plausible principle I started the post with, Jim and Sally are equally well off with respect to pain. But on the other hand, Jim has ten subjective years of pain while Sally only has five. Clearly, Jim's life is worse pain-wise than Sally's, even though at every time at which Jim suffers, Sally suffers equally, and vice versa.
Here are some interesting ways out.
- Time is discrete. Then we can't really speed up a life by a factor of two--we'd have to skip every second "frame".
- There can't be any literally constant pains. Instead, pains register at discrete moments of mental life. If these moments are sufficiently closely spaced, the pain seems continuous. On this discrete moment theory of pain, Jim will have to have twice as many moments of pain from 2010 until 2015 as Sally does, so it's false that at every time at which Jim has a pain, so does Sally.
- Pains attach to intervals of mental life rather than particular moments. This makes the equivalence principle I started the post with make no sense, since there is no such thing as having a pain at a particular time, just as the Zenonian teleportation argument can be read as teaching us that there is no such thing as moving at a particular time.