A Basic Limiting Principle (B.L.P.) is a general principle that limits our explanations metaphysically or epistemologically, and which normally goes unquestioned or even unnoticed in our everyday or scientific thinking. The term was introduced by the philosopher C. D. Broad in his 1949 paper "The Relevance of Psychical research to Philosophy": Broad offers nine examples of B.L.P.s, including the principle that there can be no backward causation, that there can be no action at a distance, and that one cannot perceive physical events or material things directly, unmediated by sensations.