Bertrand Russell referred to several different definitions and philosophical analyses as providing "logical constructions" of certain entities and expressions. Examples he cited were the Frege/Russell definition of numbers as classes of equinumerous classes, the theory of definite descriptions, the construction of matter from sense data, and several others. Generally expressions for such entities are called "incomplete symbols" and the entities themselves "logical fictions". The notion originates with Russell's logicist program of reducing mathematics to logic, was widely used by Russell, and led to the later Logical Positivist notion of construction and ultimately the widespread use of set theoretic models in philosophy.