Back in the very beginnings of my scientific education, I was taught the ‘MRS GREN’ code of life (movement, respiration, sensitivity, growth, reproduction, excretion and nutrition). Although quite rudimentary, this was a perfectly satisfactory way of distinguishing between things that were living, and things that were not.
The S in MRS GREN, sensitivity, stands for the ability of all living things to detect changes in their environment. To do this we use our senses, a characteristic dating back to virtually the beginning of life. Even simple, single celled organisms can sense their environment and respond to it.