. The paper argues why it is impossible to naturalize perception, cognition and language, i.e. to reduce to or explain in physical or biological terms that as persons and language users we may perceive, have knowledge of and a language in which we may put propositions which may be true - or false - about reality, ourselves and the situations in which we find ourselves. The inevitable circularity is first shown of any attempts empirically to account for our perception of things in the world in terms of functional processes of our brains, and their causal connections to physical stimuli emitted by things in the world. Secondly, the inevitable absurdity is shown of any attempts to account for the logical properties of intentionality and truth of cognition and language in terms of something more fundamental or elementary, which does not imply the existence of cognition, language, and these logical properties. Thirdly, it is argued that both the circularity and absurdity arises as a consequence of inconsistencies in the assumptions underlying Naturalism, among them the assumption - widely accepted since Descartes - that reality may be divided into a mental and a physical part or "realm", the content of each of which may be determined independently of referring to the content of the other.