In this second episode of a four part series on the work of Thomas Kuhn, I look at his idea of semantic incommensurability. Semantic incommensurability as applied to science for Kuhn centers around the fact that the meaning of particular scientific terms change over time. These changes become radically different as scientific paradigms shift. 'Bile' and 'planet' meant something different for Aristotle and Ptolemy than they do for us. In this episode, I go through some examples of scientific conceptual change to try and get at the meaning of it all.