[ohf]

Viruses are more like robots. They don't respond to stimuli. They just, do.

The definition of "life" comes down to splitting hairs. We don't consider something "alive" if its behavior doesn't change when its environment does.

[felipemnoa]

>>Viruses are more like robots.

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.

[dmurray]

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.

[pluma]

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).

[ravenstine]

Don't bacteriophages have to respond to some stimuli in order to attach itself to a host cell and inject genetic material?

[zaarn]

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.

The outer hull of the phage doesn't do a lot.

[iopq]

cyanobacteria don't respond to much either, they just have mechanisms to "respond" to their environment much like a robot

for a bacteria the most common response to environmental change is to die

[nyc111]

>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.

[cobbzilla]

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.

[dkersten]

So it’s more like “ability to respond to changes in the environment”.

[taneq]

Robots respond to stimuli within their construction / programming. It's what distinguishes them from dumb mechanisms.

Anyway, the term "life" is poorly and inconsistently defined and really only means "what we generally think of as life".