||A variety of topics are covered under this rubric. In general, most philosophical questions relating to the language of science are of a broadly semantic nature, having to do with the meaning, meaningfulness or reference of scientific discourse about the world. The question of the meaningfulness (or cognitive significance) of scientific discourse arose in the context of the logical positivists of the Vienna Circle, who proposed a principle of verification (or verifiability theory of meaning). The logical empiricist successors of logical positivism sought to analyze the semantic content of theoretical discourse on the basis of the connection between theoretical discourse and observational vocabulary, for example, in terms of correspondence rules. In the context of the "historical turn" associated with Thomas Kuhn, N.R. Hanson and Paul Feyerabend, the idea of meaning variance (conceptual change) came to the fore, as it was argued that the meaning of observational vocabulary depends upon theoretical context, and undergoes variation in the transition between theories. The idea of meaning variance gave rise to the semantic version of the claim of the incommensurability of scientific theories. In response to the problem of meaning variance, a number of authors (e.g. Scheffler, Putnam, Kripke) advocated an emphasis on the reference of scientific terms. In the attempt to show that reference may survive theoretical change, appeal was often made to the "new" or "causal" theory of reference advocated by Kripke.