It is easier since there are a lot of theories that could sum up and provide evidence to support that an object has life than life itself.
One popular theory is the cell theory which states that all organism is made up of cells, cell that came from those pre-existing cells, and reproduce by those cells themselves.
He was studying genetics by performing mating crosses in pea plants. Then, he discovered that the traits in the offspring didn't always match the traits in the parental plants.
