Karl Popper was a 20th century philosopher of science, best known for his work on falsifiability. He was critical of the ideas put forth by previous philosophers such as Carnap, that science works by verifying your theories through examination of the world. He said that many theories that were not scientific could be successfully verified by either making vague predictions, or through ad hoc adjustments to the theory. For example a horoscope can predict something vague like “you will have a pleasant surprise later this week”. Then you find some forgotten money in your pocket, and the horoscope was seemingly verified to be true! However since nearly anything could have verified it, since it was so vague, this does not count as science.
He was particularly critical of Freud’s theory of psychoanalysis and Marx’s theory of historical materialism, both of which were considered scientific by many at the time, but seemed to explain almost all sets of observable data. Instead he suggested that scientific theories must put forwards highly specific predictions, and the scientists must then work to falsify, rather than verify, the theory.
Marx lived in a transitional period where the scientific method hadn’t fully taken hold.
Later scholars have resoundingly confirmed his economic theories with more rigorous methods, even though their focus may differ from his. For instance Piketty’s Capital in the 21st Century is incomparable for its thoroughness and absolutely trounces liberal economics.