Psychologists an scientists do not always agree on what should be considered Comparative Psychology. Taken in its most usual, broad sense, it refers to the study of the behavior and mental life of animals other than human beings. Another possibility is that the emphasis should be placed on cross-species comparisons—including human to non-human animal comparisons. Some researchers however, feel that direct comparisons should not be the sole focus of comparative psychology and that intense focus on a single organism to understand its behavior is just as desirable, if not more. Donald Dewsbury reviews the works of several psychologists and their definitions and concludes that the object of comparative psychology is to establish principles of generality focusing on both proximate and ultimate causation. It has been suggested that the term itself be discarded since it fails to be descriptive of the field but no appropriate replacement has been found. If looking for a precise definition, one may define comparative psychology as psychology concerned with the evolution (phylogenetic history and adaptive significance) and development (ontogenetic history and mechanism) of behavior .