Life is highly instructive.
Not my own, personal, human life - that's kind of a mess right now. I mean life as in the property shared by all living things. The "are viruses alive?" kind of life.
Life is really, really, useful. As a metaphor. Because its associated philosophical problems have been more or less dispatched, and this provides a model for how many other conundrums are going to probably, inevitably go.
To recap: What's the problem with life?
Did you know that Louis Pasteur was a proponent of vitalism? Yes, the same Pasteur who got a glassblower to make him a swan-neck flask so he could show that it was particles in the air, not air itself, that caused broth to ferment. He was convinced that fermentation was such an idiosyncratic marker of life that it was not reducible to ordinary chemistry.
Pasteur was not a dumb guy, and he didn't live all that long ago. This is important: Pasteur was not violating modern norms of thought. The way Pasteur thought about life is so compatible with modernity that we still use this way of thinking ourselves for other topics.
To Pasteur, living things had some hypothetical special property (the essence of life... itself). Things either had the special property or they didn't - if they didn't they were just boring nonliving matter, but if they had the special property then by virtue of this single property they could do diverse things like ferment broth, or synthesize urea, or reproduce themselves.
This is a very easy and natural way for people to think about life (certainly it was common among the ancient Greeks). Why? Because it's simply the prediction that our mental categories match up with the natural categories of the world. We mentally separate things into living and nonliving, and we have different habits of thought depending on this categorization. It would be so convenient if the world also separated things cleanly into living and nonliving, with different rules for each.
This easy logic works great for everyday cases - things in different mental categories will pretty reliably follow different everyday patterns. Pasteur's mistake was trying to substitute in "universal laws" for "everyday patterns." You show me someone advanced enough to think about the work in terms of laws, and I'll show you someone advanced enough to invent vitalism.
So what happened?
Consider the reasoning applied to the question "are viruses alive?". Pasteur would think about this as a question of whether viruses have that special property. They either have it and are alive, or they don't and are not. Abilities like fermentation are thought of as mere indicators for whether viruses have the special property. This is probably the right time to link to How An Algorithm Feels From Inside as relevant.
Right around the time of Pasteur, the argument between "vitalists" and "mechanists" was undergoing its big shift. There were always arguments for mechanism alongside those for vitalism, at least since Aristotle and Democritus, but what changed was a combination of both empirical and conceptual advances. The boundary between living and nonliving matter was eroded by advances like the artificial synthesis of urea, the discovery of muscles' response to electricity, and the mapping of the cell. And conceptually, developments in evolution, cytology, and even thermodynamics were being synthesized into a more detailed and satisfying version of the mechanistic account of life.
Eventually, as the early 20th century rolled around, vitalism just wasn't as convincing to people anymore. It was never disproved wholesale - people still happily maintained that there was some special property to living matter even after excluding fermentation from the special abilities of life. But the number of those people dwindled, and as people used the mechanistic picture more and more, they worried about the essence of life less and less.
These days, if you ask some sophisticated person "are viruses alive?" you'll probably get an answer that talks about how they have a lot of the machinery of life, and certainly evolve, but don't have some of the other things we normally associate with life, like metabolism. And this is widely regarded as a full and maybe even satisfying answer, even though it never actually says "yes they're alive" or "no they're not." But the spirit of vitalism lives on in those who persist in trying to find out if viruses are, after it all, really alive.
Is this about consciousness? It's about consciousness, isn't it.
Okay fine, the most obvious metaphor here is consciousness. So I might as well get it out of the way.
When we look around, we see a difference between conscious things and unconscious things, and it's very intuitive to suppose that there's some special property at work. Either you have the special property and are conscious, or you don't and are not.
Let's dig in to the life metaphor. We can construct quite a strong steelman of the modern vitalist. "Sure," they say, "we can answer all sorts of physical questions about the virus. But you're not saying whether it's actually alive. In fact, your entire way of thinking about this problem has abandoned the ability to answer the question of whether anything is actually alive! And aliveness matters! For example, I only care about animal rights because animals are alive - if they weren't alive, I wouldn't care about them. So predicting certain behaviors of physical systems is all well and good, but we also need a framework that solves the 'hard problem of aliveness,' taking a physical description (if possible) of a system and telling us not just about its behaviors, but whether it's really alive."
This is of course cribbed straight from Saul Kripke, David Chalmers, et al. talking about consciousness. I don't think they would object to this depiction of the similarities - they would just say that there are also differences, which mean that a similar way of thinking about the world is right in the case of consciousness. I'm not going to dive into these possible arguments - my goal here is, instead, to take the example of life and find the analogous way forward.
These days, we don't feel like we're missing some important fact if we don't say for certain whether viruses are alive - instead, the modus ponens has gone the other way, with edge cases illustrating how what Pasteur was thinking of as a single essence is not so unified after all. We can imagine a future where something similar happens to consciousness.
But what about important uses of the idea of "alive," like in moral decisionmaking? Well, we usually just recognize that we have some commonsense notion of life, based on a cluster of correlated properties, that's good enough in everyday cases. If there's some crucial decision where it's important to be precise (e.g. what counts as a sign of extraterrestrial life), we would try to break the decision down into physical patterns that we care about, rather than holding fast to a decision rule in terms of "aliveness" and trying to be precise about which edge case have the special property. This could as well be done for "consciousness" as for "aliveness."
But this has somewhat wider application.
This kind of imagination exercise is really what this post is about. We have plenty of properties that we think of as special properties that attach themselves to objects in a physically implausible way, rather than in terms of a fine-grained explanation. If this special property starts to cause trouble, life is an excellent paradigm case to refer back to for how we might navigate an ordinary physical world.
Some more example distinctions, of varying degree of seriousness:
Intentional / Unintentional actions
Food / Not food (More generally, lots of things related to Purity / Contamination)
Free-willed agents / Un-free actors
Legal / Illegal (I'm trying to find a great old blog post about a professor assigning two students to research different sides of a complicated case, and both students being convinced that the case had a uniquely right side [theirs, of course])
Me / Everything Else