In psychology, rigidity refers to an obstinate inability to yield or a refusal to appreciate another person's viewpoint or emotions characterized by a lack of empathy. It can also refer to the tendency to perseverate, which is the inability to change habits and the inability to modify concepts and attitudes once developed. A specific example of rigidity is functional fixedness, which is a difficulty conceiving new uses for familiar objects. Systematic research on rigidity can be found tracing back to Gestalt psychologists, going as far back as the late 19th to early 20th century. With more than 100 years of research on the matter there is some established and clear data. Nonetheless, there is still much controversy surrounding several of the fundamental aspects of rigidity. In the early stages of approaching the idea of rigidity, it is treated as "a unidimensional continuum ranging from rigid at one end to flexible at the other". This idea dates back to the 1800s and was later articulated by Charles Spearman who described it as mental inertia. Prior to 1960 many definitions for the term rigidity were afloat. One example includes Kurt Goldstein's, which he stated, "adherence to a present performance in an inadequate way", another being Milton Rokeach saying the definition was, "[the] inability to change one's set when the objective conditions demand it".