Below, some of the main criticisms of falsificationism are briefly explained.
(1) Some legitimate parts of science seem not to be falsifiable
These fall into some categories.
(a) Probabilistic statements
Science often seems to issue statements about the probability of some occurrence. For example, modern physics tells us that the half life of uranium 235 is 710,000,000 years, which means that the probability of one atom of uranium decaying in 710,000,000 years is one-half .
However, such statements cannot be falsified because an experiment may produce an improbable outcome and that is consistent with the original statement – improbable things are bound to happen sometimes. Any statement about the probability of a single event is not falsifiable, so, for example, the probability that a particular coin toss will land heads is 1/2, but we cannot falsify that hypothesis by tossing the coin because the fact that the probability is 1/2 is consistent with the coin landing heads or tails on that occasion.
(b) Existential statements
Although Popper is right that a universal generalisation can be falsified by just one negative instance, many statements in science are not of this form. For example, scientific theories assert the existence of things like black holes, atoms, viruses, DNA and so on. Statements that assert the existence of something cannot be falsified by one’s failure to find them.
(2) Falsificationism is not itself falsifiable
Popper admits this but says that his own theory is not supposed to be because it is a philosophical or logical theory of the scientific method, and not itself a scientific theory, so this objection, although often made, misses its target.