Photo from Wikipedia

Everything that is alive has to respond to changes. This is called a “stimulus response”; life needs this. One kind of response would be equivalent to plants’ leaves turning to capture more sunlight for photosynthesis.

This response is usually made by movement or chemical changes and can keep a steady balance in a healthy environment.

But, this response isn’t just a meaningless juncture of movement or changes, no, this response requires something unique but still can not define life by itself.

See, if you had water, which is not alive, by the way, and you tried to make it respond to stimulus . . . it would do it. For example, let’s say our water is peacefully running down a river. Then, suddenly, a boulder falls from a nearby slope and jams up the river. Now, is the water going to stop flowing? Is it going to say, “oh well, I can’t respond to stimulus so I cannot move forward.” No!

This water will eventually make its way around or above the rock. This required a response to stimulus. Now, the water isn’t alive, but things that are alive require stimulus. If it cannot respond to stimulus than it is not alive.

Response to stimulus is just one aspect of life, you need all seven, remember?