This book offers a two-track treatment of leading 20th-century conceptions of what language is. Track One reviews the respective conceptions portraying the essence of language and language as material, behavioural, mental, biological, social, abstract and autonomous. Each conception is analyzed to lay bare the beliefs making up its content, the justification offered for it, the conceptual origins it springs from, and also the merits and flaws it has. The analyzes focus on the relevant thought of a wide variety of distinguished linguists, philosophers and psychologists, including Bloomfield, Chomsky, Dummett, Fodor, Katz, Labov, Popper, Quine, Sapir, Saussure, Skinner and Wittgenstein. Consideration is given throughout to how scholars have set about constructing and critically appraising conceptions of language. It concludes by subjecting the analyzed conceptions of language to a comparative appraisal. Track Two unfolds a humour-coated account of the ways in which the various conceptions of language have been promoted on The Metaphysics Market.