According to the Oxford English Dictonary George Berkeley introduced the term a priori into English. His inspiration for this was, it seems, to be found partly in the writings of his immediate predecessors, particularly Pierre Bayle, and partly in his pedagogical work where he adjudicated disputations between his pupils. Some of his arguments against the existence of matter Berkeley tells us are a priori, others a posteriori. Even the a priori arguments are underpinned by prior semantic principles of an anti-abstractionist character, which are shown to be important particularly in the immaterialist philosophy of mathematics. Berkeley's courageously unorthodox, and generally unpublished, thoughts about mathematics thus grow from the same soil as his celebrated denial of matter.