What criteria do we use to decide whether something is alive?
We tend to think of visible movement, either growth-related or not, as common evidence for being alive. For example, we see a dog running or observe breathing in sleeping organisms. Plants show growth over time, indicating life.
However, visible movement alone is not enough as a defining characteristic. A plant not visibly growing is still alive, and some animals breathe without visible movement.
The more fundamental criterion is molecular movement. Living organisms are well-organised structures that must constantly repair and maintain themselves. Since structures are made of molecules, molecular movements are essential for life. This explains why viruses, lacking molecular movement, are controversial regarding whether they are truly alive.
Explanation
The textbook explains that while we commonly use visible movement as evidence for life, it is insufficient as a defining characteristic. The more scientific criterion is molecular movement - because living organisms are organised structures that constantly need repair and maintenance at the molecular level. The context uses the example of viruses to illustrate this point: viruses do not show molecular movement until they infect a cell, which is why there is controversy about whether they are truly alive.