What is life? We all know a living thing when we see it, but when you get right down to it, it’s hard to come up with a consistent definition. For instance, plants grow and are alive, but crystals, which also grow, are not alive. What is the difference? In 1944, the physicist Erwin Schrödinger presented a very simple definition of life that has withstood the test of time. He identified one property that all life shares: living things avoid decay into equilibrium.