I bet every biology student has struggled with the introductory "what is life" chapter, full of conflicting definitions.<p>It is so difficult to define life because there is nothing to be defined. There are complex stuff and simple stuff. We tend to call things that exhibit a certain degree of complexity "alive", but that's just a word.<p>Can't remember who wrote the perfect definition of Biology: "The study of complex things". There's a great deal of wisdom in that.
What if we could use the notion of entropy in a definition of life?<p>Living things use energy to prevent entropy (decay?) while they are alive.<p>On the other hand, natural, non-living processes do no such thing, and increase entropy as governed by the law of thermodynamics.<p>We could also use criteria such as growth, reaction to stimuli, and reproduction. However, crystalline structures also grow and chemical processes may produce more of a certain molecule. All of those seek some energy minimizing equilibrium without any mitigating mechanisms.
I never understand why viruses are not life. Bacteria are life because they are essentially a single-cell organism and act like any cell in our body: they consume energy and produce waste. Viruses may not be a cell, but they can be damaged, they die, and they appear to be more than a simple chemical process.
Trying to define "life" is one of the central questions of the philosophy of biology. And the positions one takes in the philosophy of biology are inevitably going to be influenced by one's metaphysical positions regarding the problem of universals. A nominalist, a Aristotelian realist, a Platonist (etc.), are all going to approach the question "what is life?" differently.<p>Sometimes when people discuss this topic, they assume a certain approach to metaphysics without making that assumption explicit (indeed, they may not be aware that there are genuine alternatives to their personal metaphysical positions.)
I consider legal entities such as corporations to be living organisms. So I don't agree that life is limited to organisms with a visible (biological) body.
Here's a controversial view, but any good comments section needs a few of them:<p>Viruses are no more "life" than bacteria are "life" or amoeba are "life" or ants are "life" or mice are "life" or humans are "life"... We're all just chemical machines, nothing more nothing less.<p>Giving the word 'life' some special significance is misleading and confusing. In order to not confuse ourselves we should be talking about functions and descriptions of those functions as implemented among chemical machines.<p>Do viruses reproduce? Yes, by hijacking cellular machinery. Do bacteria reproduce? Yes, by cloning themselves. ...<p>This sort of thinking about the world allows us to stay close to reality without abstract and ultimately meaningless distractions like 'life'. Heaving the burden of abstraction onto 'reproduce' is still dangerous, but much less so than allowing 'life' to lurk as an abstract category in our minds.
1 micron is amazingly large. 300nm is about the threshold to resolve a particle under light microscopy, so these things would just be there - you could use a pipette and push them around.
anyone hear the theory that cytomegalovirus is a fragment of a gigantic prehistoric virus? a nurse friend heard about the theory in his medical microbiology class but I haven't been able to find much online.
My personal definition of life: self-replication with unlimited heredity.<p>self-replication: something can make a copy of itself.<p>unlimited heredity: when "information" is added, it is copied along, even if it does not contribute directly to the replication process itself.<p>Implied in the definition is variation. And in the bigger view, that all replicators are under pressure to do so well, or be outcompeted on resource usage etc., and disappear.<p>While a virus depends on a rich and active environment to work, they clearly fall into that definition.
Wasn't it always taught that an active metabolism separates living from nonliving?<p>This brings up other questions though about how complex a virus can be. And if they can be arbitrarily complex, will we rethink our definitions?
"Life. Don't talk to me about life." - Marvin.<p><a href="http://hitchhikers.wikia.com/wiki/Marvin" rel="nofollow">http://hitchhikers.wikia.com/wiki/Marvin</a>