PSYCHOLOGY is generally considered to be the science of mind, although more properly it is the science of mental states—thoughts, feelings, and acts of volition. It was formerly the custom of writers on the subject of psychology to begin by an attempt to define and describe the nature of mind, before proceeding to a consideration of the subject of the various mental spates and activities. But more recent authorities have rebelled against this demand, and have claimed that it is no more reasonable to hold that psychology should be held to an explanation of the ultimate nature of mind than it is that physical science be held to an explanation of the ultimate nature of matter. The attempt to explain the ultimate nature of either is futile—no actual necessity exists for explanation in either case. Physics may explain the phenomena of matter, and psychology the phenomena of mind, without regard to the ultimate nature of the substance of either.