Our belief in any particular natural law cannot have a safer basis than our unsuccessful critical attempts to refute it.
Falsification should be applied to any idea about how the physical world works. Any hypothesis about the physical world should be falsifiable. The definition for 'hypothesis' I give my students is:
A predictive statement that through experimental investigation could be shown to be false.
Applying falsification to the non-physical world - eg politics and other human affairs - is a little more problematic. However, it's always worth having the idea of falsification in the back of your mind when someone proposes an explanation for something.
When a claim is not falsifiable, you are dealing with an Immunised Hypothesis. Claims about the physical world that are based on immunised hypotheses are pseudo-scientific. Examples of pseudo-sciences that Popper became suspicious of are psychoanalytic beliefs such as those of Freud and political Ideologies such as Marxism. These theories can never go wrong, as they are sufficiently flexible to accommodate any type of new behaviour. No observation or test can show these theories to be false, as their proponents are able to invent "just-so" stories to account for any possible behaviour. These theories give the appearance of being able to explain everything, but in fact they explain nothing, as they can rule out nothing. Of course there are other pseudo-scientific beliefs we can easily include with the previously mentioned ones - intelligent design, astrology, fortune telling, tarot cards…
In this podcast we discuss Falsification and Immunised Hypotheses.
Here's the link to direct download: Tutorial 02 Falsification.mp3 (20 mins & 18mb) and you subscribe here: http://huntinghumbug101.podbean.com/feed/.
Tagged - Fallacy Skepticism, Science, Philosophy, Falsification, Hypothesis, Theory.