Of course it is usually possible to distinguish living things from non-living, though there are grey areas. Are viruses living things? - It depends on how you define 'life'. Viruses are alive, but parasitic on animals and plants. If there were no animals or plants there would be no viruses. Therefore, the definition of life must include or exclude the word 'independent'. I've always considered viruses 'conditionally'alive. In considering the origin of life, researchers have always looked at the simplist bacteria, pleuropnemonia organisms and such, independently able to stay alive, by producing their own endogenous energy, and reproduce.