Leśniewski's name has often been mentioned in earlier chapters of this book. This reader can see, however, that Leśniewski took no direct part in the research conducted in the Warsaw School and discussed in Chapters V and VI. The equivalential sentential calculus was the only exception. Leśniewski's logical production formed in a sense a separate trend in the Warsaw School. It should be noted that Leśniewski expressly repudiated his earlier opinions, that is those dating from 1911–6.¹ But he was not convinced that the shape of mathematical logic in the version to be found in Principia Mathematica was the correct one. His cutting himself off from the "grammatical" style in logic and doubts about the precision of the standard approach to logic led Leśniewski to search for a system of logic that would satisfy two general requirements: it should serve as the foundation for mathematics and should be constructed in a manner free of any ambiguities.