Living things are made of one or more cells. This is the least disputed criterion, and it is what excludes viruses.
Step 1: Something to Notice
Watch first. The explanation comes later.
A virus has genes, evolves by natural selection, and is shaped by its environment. It has no metabolism, cannot reproduce alone, and can be crystallised like a mineral and stored on a shelf.
Is it alive, and does the question have an answer?
Step 2: Find Out
Apply each criterion to fire, a virus, and a bacterium. Isolate which single criterion does the most work in separating them.
Step 1 — Predict
Which criterion best separates a virus from a bacterium?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
Metabolism
Living things run chemical reactions that release and use energy. This criterion does most of the separating work.
Metabolism The chemical reactions in an organism that release and use energy.
Homeostasis
Maintaining stable internal conditions against a changing outside — temperature, water, pH, concentration.
Homeostasis Maintaining stable internal conditions despite external change.
Reproduction and heredity
Producing offspring that inherit traits. A virus fails this: it directs a host to copy it rather than copying itself.
Growth, response, and evolution
Living things grow, respond to stimuli, and populations of them change over generations.
A cluster, not a definition
The criteria work together for typical cases. Fire meets several and a virus meets some, which is why edge cases stay disputed.
Every definition of life fails somewhere
Lists of characteristics — metabolism, growth, response, reproduction, homeostasis — exclude mules and include fire. Viruses satisfy some and not others. The category has fuzzy edges, and that is a fact about the world rather than a failure of the definition.
Life is a property of organization
No single molecule in a cell is alive. The property emerges from the arrangement and interaction of parts, which is why taking a cell apart and reassembling it does not work.
Step 4: A Common Mistake
Lots of people think
“There is a clean checklist that separates living from non-living.”
Step 5: Why the Edge Cases Are Not a Failure
The same idea somewhere new.
Species, gene and even individual are all categories with disputed boundaries. That is what you should expect from categories describing things produced by a continuous historical process — evolution does not respect the divisions we draw. A definition that worked perfectly at every boundary would be evidence that the category was invented rather than discovered.
Step 6: Think It Through
Practice makes it stick.
Fire
Problem 1 of 2
Fire grows, consumes fuel, produces waste and spreads. Which criterion most clearly excludes it?
The Crystallised Virus
Problem 2 of 2
A virus can be crystallised and stored for decades, then infect a cell. What does that show?
Where the Line Falls
1 of 5
Which criterion most clearly excludes viruses from life?
2 of 5
What is homeostasis?
3 of 5
Which of these does fire satisfy?
4 of 5
Why do biological categories have disputed boundaries?
5 of 5
Does a virus reproduce itself?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Is there a clean checklist separating living from non-living?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Argue for or against classifying viruses as living, and state what your answer implies about biological categories.
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Living things are cellular, metabolise, maintain homeostasis, reproduce, grow, respond and evolve.
- Metabolism and cellular structure do most of the work at the boundaries.
- Fire satisfies several criteria; viruses satisfy some and fail others.
- The criteria are a cluster rather than a definition, which is normal for biological categories.