Science writer Ferris Jabr in the NYTimes:
You might think botanists have a precise unfailing definition of a tree — they don’t. Sometimes it’s really difficult to say whether a plant is a tree or shrub because “tree” and “shrub” are not properties intrinsic to plants — they are ideas we impinged on them.Right, there is no precise threshold. And we understand a great deal about how living things function without, however, being able to create a living thing from scratch–life in a testtube. And we may NEVER be able to do so.
Likewise, “life” is an idea. We find it useful to think of some things as alive and others as inanimate, but this division exists only in our heads.
Not only is defining life futile, but it is also unnecessary to understanding how living things work. All observable matter is, at its most fundamental level, an arrangement of atoms and their constituent particles. These associations range in complexity from something as simple as, say, a single molecule of water to something as astonishingly intricate as an ant colony. All the proposed features of life — metabolism, reproduction, evolution — are in fact processes that appear at many different regions of this great spectrum of matter. There is no precise threshold.
Consciousness is probably like that as well. No precise threshold, and we may never be able to create it artificially. So what?
Meanwhile, enjoy this video of Theo Jansen's Strandbeests, which have no internal source of propulsion, but move fluidly: