In the philosophy of perception, critical realism is the theory that some of our sense-data (for example, those of primary qualities) can and do accurately represent external objects, properties, and events, while other of our sense-data (for example, those of secondary qualities and perceptual illusions) do not accurately represent any external objects, properties, and events. Put simply, critical realism highlights a mind-dependent aspect of the world that reaches to understand (and comes to an understanding of) the mind-independent world.
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Locke
According to Locke, some sense-data, namely the sense-data of secondary qualities, do not represent anything in the external world, even if they are caused by external qualities (primary qualities). Thus it is natural to adopt a theory of critical realism.
By its talk of sense-data and representation, this theory depends on or presupposes the truth of representationalism. If critical realism is correct, then representationalism would have to be a correct theory of perception.
American critical realism
The American critical realist movement was a response both to direct realism, as well as to idealism and pragmatism. In very broad terms, American critical realism was a form of representative realism, in which there are objects that stand as mediators between independent real objects and perceivers. A prominent developer of American critical realism is Roy Wood Sellars, father of Wilfred Sellars.
One innovation was that these mediators aren't ideas (British empiricism), but properties, essences, or "character complexes."
British realism
Similar developments occurred in Britain. Major figures included Samuel Alexander, John Cook Wilson, H. A. Prichard, H. H. Price, and C. D. Broad.