 One of the great philosophers of the nineteenth century, Bernard Bolzano (1781–1848) made outstanding contributions in a number of fields (for a more comprehensive survey of his thought and biographical details, see Edgar Morscher’s  article  on Bolzano in this Encyclopedia). As a logician, he focussed on foundations, developing a theoretical framework that is still very much worth studying today, so much so that a highly respected contemporary logician could (only half facetiously) write a quite positive review of Bolzano’s Theory of Science (1837) as if it had just appeared (van Benthem, 2013). This is not to say that all aspects of Bolzano’s logical work could be mistaken for contemporary productions: if he seems at times to be at home in the twenty-first century, it is equally clear at others that we have to do with a thinker who was born in the eighteenth.