- Of all its many values, the greatest must he the freedom to doubt.
- In Hawaii, Feynman learns a lesson in humility while touring a Buddhist temple: "To every man is given the key to the gates of heaven; the same key opens the gates of hell.” This is one of Feynman's most eloquent pieces, reflecting on science's relevance to the human experience and vice versa. He also gives a lesson to fellow scientists on their responsibility to the future of civilization.
- … the scientific article says, perhaps, something like this: “The radioactive phosphorus content of the cerebrum of the rat decreases to one-half in a period of two weeks.” Now, what does that mean?
- It means that phosphorus that is in the brain of a rat (and also in mine, and yours) is not the same phosphorus as it was two weeks ago1, but that all of the atoms that are in the brain are being replaced, and the ones that were there before have gone away.
- So what is this mind, what are these atoms with consciousness? Last week’s potatoes! That is what now can remember what was going on in my mind a year ago – a mind which has long ago been replaced.
- That is what it means when one discovers how long it takes for the atoms of the brain to be replaced by other atoms, to note that the thing which I call my individuality is only a pattern or dance. The atoms come into my brain, dance a dance, then go out; always new atoms but always doing the same dance, remembering what the dance was yesterday.
- The key claim in the above extract is that “the thing which I call my individuality is only a pattern or dance”. This reminded me of the claim in "Clark (Andy) & Kuhn (Robert Lawrence) - Aeon: Video - Andy Clark - Virtual immortality" that “we are patterns in information space”.
- Are Feynman and Clark talking the same language here? Are either of them correct? I’ve discussed Clark under the head of the above paper. What about Feynman?
- Feynman’s main point is equivalent to the well-known (but maybe disputed – for example, see this footnote from Snopes3) claim that all the cells in our body are replaced every 7 years – or something like that. While Snopes point out that this claim is technically false, it is true for the bulk of our cells, but not – it seems – from a careful reading of "Frisen (Jonas), Etc. - Retrospective Birth Dating of Cells in Humans" – for our neurons. So, is he right to say the we remember with “a mind which has long ago been replaced”?
- Well, firstly he is equating mind with brain – which is debatably, but a claim I’m willing to go along with. But if this is true, it puts in tension his other claim that we are “patterns”.
- What is true is that it’s the material structure and organisation of the brain (and the rest of the body) that is important, not the individual atoms – or even the individual cells – that make it up.
- However, every time we have a thought, new connections are made or strengthened in the brain, and its structure changes slightly, so – if we are to hang on to the idea that we are a persisting thing at all – neither the particular atoms nor the particular structure of our brains and bodies are essential to our persistence, provided they evolve in a “natural” way. This is the case for the persistence of all organisms, which eat and excrete, grow and do all the other things organisms do.
- Feynman admitted to being no philosopher (and thought philosophy a waste of time in any case) and admitted that scientists are just amateurs outside their domain of expertise. Also, he doesn’t express himself unambiguously on non-scientific matters. I doubt he means what he might be understood to say.
- He’s right that it’s the “dance” of the molecules, rather than the molecules themselves, that matters, but you can’t take that evolving structure away from matter altogether and say that it’s that that we are. We are not evolving abstract objects but evolving material beings.
- Two things here
- This is not radioactive decay (any more than it is in the 14C studies undertaken in "Frisen (Jonas), Etc. - Retrospective Birth Dating of Cells in Humans"), but radioactive phosphorus is used as a marker.
- However, what is signified by the fact that the phosphorus is being swapped out with a half-life of 2 weeks?
- Given the findings in the study just cited, it doesn’t look like whole neurons are being swapped out, but some of the chemicals in them are. But – again based on the paper just cited – the 14C in the DNA in the neurons is not being swapped out, but stays there for life – so I don’t know why phosphorus is different.
- But this means that it might be (or might have been) the case that the material structure of a neuron remains the same, but not the physical atoms that make it up.
- If the structure varied capriciously, we’d not be able to remember anything, or perform actions that depend on learned or innate abilities.
- There are more interesting passages than the one selected, of course, but I was particularly struck by this one, for the reasons given above.
- Snopes: Does The Human Body Replace Itself Every Seven Years?
Claim: Every cell in the human body is replaced every seven years.
Rating: False. While the vast majority of cells would be replaced every seven to ten years, some cellular outliers make such a statement pointedly false.
- The cells within a human body are myriad, multi-functioned, and completely distinct from one another. Different kinds of human cells replace themselves at different rates, which means the human body is comprised of cells of many different ages.
- The vast majority of a these cells regenerate fairly rapidly, making most cells in a human body much younger than the age of the human itself. The researcher behind a ground-breaking study that attempted to date the ages of various cells in a human body, Jonas Frisen, famously estimated that the average age of a cell in the human body is between 7 and 10 years old in a pivotal 2005 paper published in Cell ("Frisen (Jonas), Etc. - Retrospective Birth Dating of Cells in Humans").
- These statements have led some to the conclusion that every cell in the human body is replaced every 7 to 10 years — a fun but inaccurate factoid commonly held as true. The central flaw in the argument is that an average of all the cells in a body masks the fact that some cells last very long times, and other cells don’t regenerate for a person’s entire lifetime, as described by NPR: It turns out that each body part has its own very distinct lifespan. The lining of the stomach, constantly under assault by digestive acid, is renewed every few days. But bones are refreshed once a decade. And there are a few parts of you that stay with you from birth to death.
- Using a method that allowed for radiocarbon analysis of individual types of cells, Frisen and his colleagues quantified the age of intestinal cells, skeletal muscle cells, and the gray matter of two brain regions — the cerebral cortex and the cerebellum. Compiling data from multiple individuals, they found:
- The average age of all cells in the intestine was 10.7 years, with epithelial cells (which form the outside of organs and blood vessels) being replaced on average every 5 days compared to non-epithelial cells being replaced an average of every 15.9 years.
- The average age of skeletal muscles was 15.1 years.
- The average age of cells in the gray matter (cells that make up neurons and other brain matter) of the cerebellum was almost as old as the individual, implying they form when a person is around two years old and remain with them throughout that person’s life.
- Cells from the grey matter of the occipital-cortex gray matter, however, turnover at a much higher rate.
- As NPR also noted, some other cells will also remain with you literally from embryo to death. The cells that make up the central core of the lens of an eye remain with a human from their genesis during embryonic development to their demise at death.
- Because of these outliers, there will never be a period of time over which one can accurately say that all of the cells in your body have been replaced by new ones (even if a vast majority of them will have been replaced in that time period). Therefore, we rank the claim that a human’s body is replaced on the cellular level every seven years as false.
→ Alex Kasprak, Published 30 April 2018
- I think the Snopes article has failed to pick up an important point about neurogenesis from "Frisen (Jonas), Etc. - Retrospective Birth Dating of Cells in Humans".
- While they are right to say that the apparent age of the cells making up the grey matter outside the cerebellum is much less than the age of the person, that is because this is based on the average age of the neurons and the glial cells (see Wikipedia: Glia). It seems that the neurons themselves are unchanged from age two, or thereabouts, as with the cells in the cerebellum.
Text Colour Conventions (see disclaimer)
- Blue: Text by me; © Theo Todman, 2020
- Mauve: Text by correspondent(s) or other author(s); © the author(s)