What is "Life" anyway?

In my last post, I proposed that we need to re-examine how life emerged from non-life some time in the past. But what do we mean "life"? What trait distinguishes the living from the non-living? If we are going to investigate how life emerged, we need to understand what it is we are investigating. Despite the broad use of the terms and apparent self-evidence of a definition of "living", it is, in fact, very difficult to state with scientific rigor a definition that does not bring with it, some presupposition about the nature of living things.

In the past, something was only considered to be alive, if it was able to move on its own. This eliminated the entire plant word from consideration. In a similar vein, respiration, the consumption of food and production of waste, seem ill adapted, as this eliminates viruses, which are arguably, one of the most virulent forms of life on the planet.

I propose a very simple definition: "A system is considered to be a living system, if in the general case, members of the system class possess the ability to produce another distinct living system." Despite the obvious self referential nature of this definition, it is the very reproductive capability of life that makes it unique. All other chemical processes pale in comparison to the process by which one living thing creates another distinct version of itself.

The definition makes a few very important distinctions. First, the general case for a class of systems (individuals sharing a set of traits) is key to whether or not a system can be considered living. If the normal state of members of the class is the ability to reproduce, then an individual within the class which cannot reproduce may be considered living by virtue of its membership in the class.

Second, no requirement exists for a living system to produce a copy of itself or even of a system similar to itself. All that is required is that the system produce another system capable of producing another system. In this way, both evolutionary and species stable models of life's origins can be considered.

Lastly, this definition implies nothing about the source of the system. Whether the system was produced by naturally occurring means, or as the result of intelligent, directed efforts (e.g., man-made), it has no bearing on consideration whether the system is living. Some may believe that this omission invalidates the definition, but I would argue that understanding living systems that are designed is critical to understanding living systems that exists apparently without a designer.