Karl R. Popper in Logik der Forschung , advanced the theory that the test of an empirical system, the demarcation of the limit of scientific knowledge, is its 'falsifiability' and not its 'verifiability,' his aim being "to select the one which is by comparison the fittest, by exposing them all to the fiercest struggle for survival". To be falsifiable systems of statements must be logically precise and unambiguous, i.e., capable of being "subjected to tests which might result in their refutation".