For E. Gellner, `critical' monism (the attempt to restore intellectual order by lucid principles) is necessary in our search for the truth, for validity of our knowledge about the world.
Cognitive legitimacy can be moored on different anchors, of which two selector theories are the most important: empiricism and materialism. For the empiricists, man is not more than the totality of his experiences. He is a bundle of sensations, a ghost. For the materialists, the world is a regular and constant machinery, which produces all events. A materialist looks for theories which explain the workings of the machine.
I. Kant analyzed both viewpoints and was gripped by two fears: one, that the mechanical vision (science) did not hold, and two, that it did hold. He thought that science (determinism) was right. But a new problem arose: what with human freedom and morality? Kant's solution was that we, humans, imposed the mechanistic causal order on the world. This order was not in the nature of things, because we cannot know the real nature of things. (For a fundamental critique of Kant, see W. Heisenberg `Physics and Philosophy')
A fatal blow was given to empiricism by N. Chomsky, who proved that human beings possess innate ideas