The answer to this question is as much philosophical as scientific, but as Dr Michael Cunliffe explains in a popular science article, even the most apparently obvious boundary isn’t as clear-cut as it seems.

http://www.popsci.com/science/article/2012-04/fyi-glass-really-half-full

For more information about Dr Cunliffe’s research, see the Marine Microbial Biogeochemistry Group web pages.