Logical positivism and the closely related logical empiricism were philosophical movements and theories of knowledge focused on the justification of belief through direct observation and logical reasoning. Philosophically, logical positivism centers on the verification principle of meaning, which is the position that theoretical statements about social behavior are only meaningful when they are either empirically verifiable, or logically true. In service of this principle, logical positivists were concerned with the development of rigorous scientific methods capable of measuring social behavior and testing and verifying social theories or laws. During the 1950s and 1960s, geographers began a sustained engagement with the foundational ideas of positivism leading to the quantitative revolution and the emergence of spatial science. The ensuing development of quantitative approaches to human geography, and the debates and critiques that accompanied that development, shifted the discipline toward the creation of theory and an examination of its epistemological foundations.