Researcher Christopher Voigt of the University of California, San Francisco, who works on synthetic biology, said: "We don't have a very good definition of life. It's a very abstract thing, what we call life, and at what point we say something doesn't have the necessary components versus it does, it just becomes way too murky." The question of what constitutes life has dogged scientists since the early days. Aristotle was the first to attempt to define life, and his proposal boils down to life being something that grows and maintains itself (he called this "nutrition"), and reproduces.