DATUM, literally "that which is given." The term is com monly used for anything that is regarded as beyond question or that is allowed to be assumed or taken for granted (though it may be only for the sake of argument) in connection with any problem involving further construction or interpretation or inference. Usually there is no difficulty in distinguishing the data of a prob lem from the further constructions or inferences. Sometimes, however, complications arise. It is, e.g., a common tendency to regard all perceptions as data (or facts, as they are also called). But, as is obvious in cases of illusion and hallucination, even com paratively simple percepts (like the recognition of an object or location of a sound) involve mental constructions which may be wrong. And so even in the case of percepts it becomes important to distinguish much more carefully than is commonly done the sense-data (such as mere sensations of sight or sound) from the mental constructions immediately and unwittingly put upon them.