One of my new hobbies during this pandemic is baking, and my greatest companion in this has been my sourdough starter, Dolan Junior Junior. Dolan Junior Junior's namesake is Dolan Junior, the sourdough starter DJJ split off from (my mother-in-law's) in May or so, who in turn is named for Dolan the cockapoo who died of mysterious circumstances in 2017.
A starter is an active combination of flour and water, with a healthy level of yeast and bacteria making it 'sour.' These are the living ingredients, while the flour and water provide the right environment for them to flourish (in addition to temperature and room humidity). A starter has to regularly have its waste discarded, and be refed with fresh flour and water. I feed DJJ about once a day, casting off about half of the total starter and adding roughly a half cup of flour and 1/4 to 1/3 cup water. These amounts can vary, and I'm not too precise with my measurements, but DJJ is resilient and can handle small variations in the amount that gets discarded and the amount/proportion fed.
A sourdough starter is simultaneously a living individual and a complex system. It's an individual in that it has a perceivably stable, singular physical presence. Some people might have a hard time attributing the same to, say, an anthill or a church congregation (preferring to see them as groups of individuals). What seems to undermine the starter's individuality is the rate at which it reproduces itself. From day to day, if only 50% of the physical matter of Dolan Junior Junior persists, then over seven days, less than 1% (statistically speaking) of the starter I see in front of me was there the last week. I also eat and produce waste, but it would take much longer for me to consume and discard my body weight's worth of food and waste. In addition, much of the food I eat is turned into energy, rather than converted into body mass, so it takes even longer for my body's cells to turn over. Even remembering that the flour and water added to DJJ daily are simply the environment for yeast and bacteria, it's still the case that half of those living cells would get discarded every day. It's tempting to think of the yeast and bacteria as the part of the starter which is 'actually alive,' but that would be like trying to isolate the heart, brain, or some other organ as the part of the human body that's 'actually alive.' It's in the combination of parts and their interplay that the system emerges as an individual entity.
The Ship of Theseus is a thought experiment in the philosophy of identity that asks: If the parts of a ship are gradually replaced over time, at the point at which all parts have been replaced, is it still the same ship? I encountered it in an intro philosophy class in college and don't have a good memory of how I handled it at the time. Now, though, with a complex systems perspective, I can readily answer it in the affirmative: It is undoubtedly the same ship. The question of 'sameness' the question seems to imply is that it an object is reducible to its constituent parts — absurdly, that I'm only the same person from moment to moment when all the cells in my body are exactly the same. Counterexamples to this are trivial — certainly it's the same person before and after receiving a haircut, going to the bathroom, or cutting their fingernails. It seems intuitively troubling, however, that identity can persist over decades, when it's pointed out that none of the matter in one's body remains. But when we think of identity as an emergent outcome of an arrangement and ongoing interrelation of parts (none of which are individually irreplaceable), then we can see that it outlasts the specificity of its constituent parts. It's stable, with few dramatic changes at any given moment (although it may receive internal or external shocks), and fluid over time. Mathematically speaking, we might think of identity as a smooth derivative of some choppy, higher-level function — ironically, though, that metaphor works in reverse in terms of the directionality of levels (emergence generally being thought of as a higher level and derivation as going down in levels).