There is no - and can be no - definition of 'life' from the vantage-point of life whatsoever: the twilit, purgatorial status of the virus demonstrates as much.
Erm, well actually he's right, at least from a purely scientific standpoint. Science can tell us things about biological processes, atoms, molecules, and chemical reactions, but it cannot in any rigorous way determine by itself a standard for the definition of a life. Science, can, if given a definition of a life externally, apply it. But it cannot create a definition by itself.
Science has taken some set of criteria to distinguish things that are alive from things that are not, viruses being the only ambiguity. And from this definition embryos are clearly alive. Eggs are alive, sperm are alive, skin cells are alive, etc. But the criteria were determined arbitrarily rather than empirically and are not in and of themselves a result of science, only a definition to categorize empirical observations of organic matter. Furthermore, if we take the generally accepted definition of "alive", which virtually everyone does except with the ambiguity of viruses, the missing step pertinent here is that science is not capable of defining what constitutes a singular instance of life.
My personal view is that the life of a living thing begins at the point when its beginnings can be traced furthest back to and still remain a distinguishable entity, that is for humans, at fertilization.
