The Ship of Theseus is a classic thought experiment in Greek philosophy. As related by Plutarch the historian, Theseus' ship was kept as a famous artifact, and, over the years, as its wood decomposed the boards were replaced, until none of that ship was the same wood as the original.
Was that ship still the ship of Theseus after all those replacements? Heraclitus, who said all was change, would likely have said not the same ship, and Plato and Aristotle, with their emphasis on form as what makes things what they are, might have thought otherwise. In a materialistic society, there are direct implications of the story for questions of identity over time--including human identity.
In The God Delusion, Richard Dawkins quotes, with apparent approval:
Steve Grand points out that you and
I are more like waves than permanent 'things'. He invites his reader
to think . . .
... of an experience from your childhood. Something you
remember clearly, something you can see, feel, maybe even
smell, as if you were really there. After all, you really were
there at the time, weren't you? How else would you
remember it? But here is the bombshell: you weren't there.
Not a single atom that is in your body today was there
when that event took place . . . Matter flows from place
to place and momentarily comes together to be you.
Whatever you are, therefore, you are not the stuff of
which you are made. If that doesn't make the hair stand
up on the back of your neck, read it again until it does,
because it is important.
This is a quote from Grand's book on supervenience in nature. Though I'm not disagreeing here with such a supervenience thesis, the assertion above that "Not a single atom that is in your body today was there" (when you, now an adult, experienced a past, childhood event) is, in fact, false.
What evidence do Grand and, via Grand, Dawkins, have for such an assertion?
Their outdated, incorrect claim is in fact a philosophical urban legend, where everyone vaguely quotes a paper written over 60 years ago by Dr. Paul Aebersold, then a physicist at Las Alamos with Ernest Lawrence, as part of a review of the then new technologies using radioisotopes made possible by peaceful nuclear technologies. Writing in 1953 in the Annual Report of the Smithsonian Instituion, Aebersold says,
Tracer studies show that the atomic turnover in our bodies is quite rapid and quite complete. For example, in a week or two half of the
sodium atoms that are now in our bodies will be replaced by other sodium atoms. The case is similar for hydrogen and phosphorus.
Even half of the carbon atoms will be replaced in a month or two. And so the story goes for nearly all the elements. Indeed, it has been shown
that in a year approximately 98 percent of the atoms in us now will be replaced by other atoms that we take in in our air, food, and drink.
Aebersold's paper was later to be quoted in 1954 in Time Magazine, whence it passed into the "quoted by everyone facts" category. By the 1960's, it seems to have become embedded in the background notions of the philosophy of biology and identity, perhaps slowly taking on urban legend status over the next few decades.
Let's now update to the twenty-first century. Pulse-chase analysis is a technique used in biology to understand the changes in a cell or tissue over time. The cells are exposed to a labeled substance which it incorporates into cellular metabolism at the time of exposure. This "pulse" is followed by a further exposure of the same type substance, but without the label (the "chaser"). The cell or tissue can later be examined for the tracer at various times after the pulse of label, to see if and where in its metabolic flow the tracer can be located.
In the past 15 years, studies done mostly at the Frisen lab at the Karolinska Intitute in Norway took advantage of a global carbon-14 pulse-chase experiment: the global atmospheric fallout exposure of the world's population alive or in utero at the time of atmospheric testing of atomic weapons by the United States, Russia, and Great Britain from 1945 through 1963 (note that France and China did a few more such tests through 1980). Here's one of the first abstracts:
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Cell. 2005 Jul 15;122(1):133-43.
Retrospective birth dating of cells in humans.
Spalding KL1, Bhardwaj RD, Buchholz BA, Druid H, Frisén J.
Abstract
The generation of cells in the human body has been difficult to study, and our understanding of cell turnover is limited. Testing of nuclear weapons resulted in a dramatic global increase in the levels of the isotope 14C in the atmosphere, followed by an exponential decrease after 1963. We show that the level of 14C in genomic DNA closely parallels atmospheric levels and can be used to establish the time point when the DNA was synthesized and cells were born. We use this strategy to determine the age of cells in the cortex of the adult human brain and show that whereas nonneuronal cells are exchanged, occipital neurons are as old as the individual, supporting the view that postnatal neurogenesis does not take place in this region. Retrospective birth dating is a generally applicable strategy that can be used to measure cell turnover in man under physiological and pathological conditions.
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So, here is a chart of the results of many such tests, courtesy of bionumbers.org:
What was wrong with the old urban legend? Note that Aebersold was mostly right (except for that word "complete") in what he wrote: it was extrapolation of what he said that was wrong!
Even in 1953, we knew that the body was not a uniform bag of tissue, and that cells were not mere bags of solute. It turns out that although it's true that over 98% of the body undergoes relatively rapid turnover during life, there are several tissues whose DNA does not turn over nearly as fast, or in some cases turn over at all.
Most of the human body is water, and this water flows in and through us like Heraclitus' river. However, bone turnover is far slower, and some tissues, such as tendons, turn over even less. Brain tissue contains some neurons which have not divided since we were born. DNA repair in those cells is not pervasive enough to turn over much of it during our lifespan.
So, not all the atoms of the human body turn over during life. Much of our DNA lingers.
Does it actually matter to philosophical conclusions about our identity that some small part of us (possibly an ounce) is completely unchanged during our lifetimes? Not if, like most of the world's population, you know that a ship is not just deadwood, and that matter is not all there is to living.
