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.
Words are only useful insofar as they point to groups of entities that the speaker and listener agree are being pointed to. So, we should ask "what is the most useful definition of life?" It's a short word, so it's valuable real-estate. What should the definition of life be?
It's easy to get wrapped up into discussions about words, but don't forget that the things they point to is what's really interesting.
I agree. I think a source of confusion is the following.
Everyone has built up, over their lifetime, an implicit mental notion of "life", which is based upon the sorts of characteristics of things they classify as being alive. Many (I'd say most) people think this mental notion is some objective category that exists external to their mind, some thing that has a kind of essence that is shared by all the instances of life. They see the task of defining life as defining the essence of this category, and see there being an objective question as to whether something (e.g. a kind of virus) fits this category.
But this is to confuse a mental construct with what is out there. There are the objective properties of things like different types of viruses, different types of bacteria, of different types of mammals, and of all other natural phenomena. You can talk about what the similarities and differences are between the objective properties of a type of virus and a type of bacteria, for example, and there are simpler kinds of phenomena vs more complex ones, but there's no actual "life" category out there, separate from our mental constructs, to be objectively defined.
>Sb trying to push sb else to use a definition meets the second part of the sentence.
This is post-modernism in thin disguise.
Certain categorizations can be more valuable (practical, useful, just, etc) than others, in which case it's reasonable to rally for their use over other, less valueable, categorizations.
Some categories are just completely impractical, for example the category of "all objects except for my dog and its kibble, and excluding those that came in to existence on an even-numbered Gregorian year." Language is an application of signaling theory and there are lots of statements we can make about it. (For example it would be a bad idea to re-define the word "good" to mean that one above.)
It's post-modernism only if you assume that human beings have no set, general psychological characteristics and are entirely the product of socialization. Otherwise it is pragmatism.
It's most assuredly post-modernist because it implicitly assumes that all categories are equal in value (i.e.: that no category is better than another), and that the decision is at it's core a power-struggle (i.e.: oppressive to the loser).
Our understanding of life, viruses, and the relation between them is incomplete which means our definitions and lexicon as they are interrelated is also lacking in resolution.
Before the discovery of these Giant Viruses as I guess we are calling them now, you used to be able to make a useful distinction between cellular-based life, and viruses. Viruses were mere packets of information as far as our best understanding went with very simple mechanics. Cells are more complex, with a useful distinction being made between prokaryotes and eukaryotes when necessary, but each could be easily identified as being made up of one or more cells.
As our understanding of life improves, so does our useful observations into that murky grey area between life and not-life, and I suspect we will have to potentially extend the definition of life but come up with a vocabulary that more usefully delimits the forms it can take from its most simplest forms into the murkier grey area where viruses start to resemble cell-based life.
Science is a work in progress, and so must be the scientific lexicon. Consider further your use of the word "die" and is that really the word you would use when a virus expires? Maybe, if you consider them to be alive in the first place, but you might use the word "destroy" if you considered them to be more like an object, or at least not alive.
From David Goodsells The Machinery of Life:
"In 1944, the physicist Erwin Schrödinger presented a very simple definition of life that has withstood the test of time. He identified one property that all life shares: living things avoid decay into equilibrium."
Neither do rickettsia or chlamydia, two bacteria. Honestly, whoever was deciding on the definition of "what is life" knew ahead of time the kinds of things they wanted to include and they worked backwards from there.
We want to exclude viruses because they are too mechanical. There's no magic to them. That's the way it appeared before we started finding all these giant viruses anyways.
So viruses are data but living things are code. Viruses are just malicious data that exploits unintended behavior of the code to replicate itself. Of course all code is just data.
It's similar to how it's difficult to argue whether a data format is a programming language (Turing completeness is an obvious but unnecessary and insufficient distinction). "Declarative programming languages" muddy the waters but most people would agree that a JPEG image is just data whereas a PHP script is a program.
Humans also survive by ingesting other life forms (plants, animals). We couldn’t survive without vegetation either, because it converts our waste products into useful stuff (e.g. CO2 to oxygen).
Ingestion isn't really reproduction though, even if the dependency is there. I'm not saying it's a perfect definition, just pointing out that it's never been "consume energy, produce waste".
but it's unclear that this definition really means anything, because you can say that humans lack the facilities to replicate itself, without access to food and water and air too!
We have the organs to reproduce provided that we, you know, exist... I'm not arguing how correct the definition is, more that it's what is 'textbook' from my understanding, and not really do do with the consume energy, produce waste comment or ability to react to an environment like some other posts suggested.
A virus is just a computer program that needs a Von Neumann machine (CPU) to run on. Following this analogy we could define alive by saying that the combination of a program and a Von Neumann machine makes the entity alive.
Except that they really do have a physical manifestation (from the article, the cylindrical tail, the polyhedral box containing the genes, the fibrils on the outside). In some way they are closer to being the USB stick containing the computer program, that can reproduce onto other USB sticks but needs a host computer to do so.
Everything[0] has a physical manifestation. A computer program has a physical manifestation on its storage medium.
[0]: Let's not argue about whether it makes sense to say that abstract concepts also have a physical manifestation. Programs are concrete expressions encoded on a physical medium (whether using magnets, electrical charges or even the position of mechanical switches).
Bacteriophages attach to certain receptors, they don't respond to any stimuli.
The entire process of injecting their RNA is simply a chemical reaction to being attached to a receptor.
Basically, they just drift along until they hit a bacteria with the right receptor, then inject their genetic material and the empty hull is either discarded or simply continues to hang onto the cell.
>We don't consider something "alive" if its behavior doesn't change when its environment does.
I think this is too vague. A cat may continue to sleep without responding to changes in its environment but may respond to an internal stimulus (hunger) and get up and go. So response or lack of response to the environment may not be a good criterion to define life.
I think you're reading this too narrowly. The cat will respond to changes in its environment, even when sleeping. Try raising or lowering the temperature by an extreme amount while the cat is sleeping -- at some point it will awaken and seek a more comfortable place. To what extent does one need to change a virus's environment before it changes behavior? If its behavior is "fixed" by its genes and well understood, then there isn't much "dynamic" (complex) behavior being exhibited.
Perhaps when we consider that viruses are unable to form complex structures like tissues, organs and entire organisms, we must rethink if they're the same level of "life" as cells? I don't think a single definition of life is useful or might even exist, just as I don't think a single universal physical theory is possible.
The thinking on this has been changing recently as more discoveries are made. The linked article even alludes to it by calling viruses the fourth domain, implying that they are indeed life.
