Among those who believe that psychoanalysis is not science is the philosopher Karl Popper. Popper holds that the demarcation criterion that separates science from logic, myth, religion, metaphysics, etc. is that all scientific theories can be falsified by empirical tests--that is, a scientific theory rules out some class of events, and if one of those events occurs, then the theory is declared false. According to Popper, psychoanalysis does not meet the falsification criterion because it does not rule out any class of events. Because it explains everything, it explains nothing.
Adolf Grünbaum disagrees with Popper. Grünbaum believes that Freud meant his theory to be scientific, that he did make falsifiable predictions, and that those predictions proved false. For example, Freud's Master Proposition, also known as the Necessary Condition Thesis (NCT), is that ONLY psychoanalysis can produce a durable cure of a psychoneurosis (a mental illness caused by childhood trauma). This is a strong statement that could be falsified if, for example, another form of therapy such as behavior therapy cured someone of a neurosis, or even if spontaneous remission occurred. We now know that neurosis yields to both of these alternatives. Therefore, Grünbaum concludes that psychoanalysis, being false, is bad science. Grünbaum's arguments are discussed in detail at Psychoanalysis
Activity: Dramatic interpretation: Question and Answer with Adolf Grünbaum.
Activity: Debate: Is psychoanalysis good science, bad science, or not science?