What makes something alive?

I realized, this morning, that I objectify yeast. Sure I objectify meat, vegetables, trees and, heck, even some people but, even when I'm doing so, I don't lose sight of the fact that these things are or were living beings. Yeast, however, I treat a great deal more like a chemical or tiny machine. Yeast is something that I add to wort to make beer; sure I need to temperature control it and keep things clean but that's the case with lots of wet chemistry. If I didn't know better, I might think of yeast as little more than a catalyst for converting sugar to ethanol and carbon dioxide; there are nuances and yeast imparts other processing to the wort but those are minor details. Continued thinking, combined with various old thoughts of mine and some of my personal philosophies led me to question the nature of life.

If I could replace yeast with a single chemical or mixture of a few chemicals that were capable of converting wort to beer, would that mixture be alive? By most definitions, probably not, but what then makes yeast alive? Is it perhaps that yeast separates its innards from the outer world? What if I made membrane bubbles filled with wort-to-beer chemicals that let reactants in and products out, would that be alive? Perhaps it's self-replication that makes yeast alive? What if I put nano-machines in the membrane bubble that were capable of duplicating themselves and the chemicals in the bubble as well as increasing the bubble size and splitting it in half? Now we've probably stepped well past the gray area and have either made something that is either alive or nearly impossible to distinguish from something alive.

What if we extend our self-replicating ethanol bubble notion? Would a self-replicating mining robot be alive? Are computer worms alive? Is a lathe that can be used to make more lathes like a virus in being almost alive, save for its need of host (lathe operator)? If I write a piece of software that simulates yeast at an atomic level, is that piece of software alive?

Of course, already following pathetic and weakly emergent hylopathism, I'm of the opinion that every example I've given, from enzyme to yeast, from lathe to myself, is alive. My hylopathic view of aliveness, however, is quite at home coexisting with conceptions of other people's definitions of aliveness in my head. I find that allowing contradictory and, possibly, mutually exclusive memes to live side by side in my head makes for some very interesting philosophizing and internal dialogs.

I'm wondering though, Internet, where do you draw your lines? What makes something alive?


The yeast may be doing a lot more than just converting sugar to alcohol. Maybe that is where it's personality resides, each yeast personality being slightly different from the next and able to preserve that personality through the same common mechanisms that other yeast personalities use.

The factors that distinguish some observed entity as "living" are...

~ it has a metabolism of some kind ~ it has DNA ~ it can gain mass or grow ~ it can reproduce or multiply ~ it can pass on traits through heredity ~ it can evolve over generations

What you list is one of many attempts to define what is alive; the central topic of this post is to raise the question of which such definition is the correct one to use.