According to Dale H. Schunk, in his book Learning Theories: An Educational Perspective, theorists and researchers do not have a fixed definition of learning . Schunk (1991) also mentions that “learning is a lasting change in behavior, or in the ability to behave in a given way, that results from practice or other forms of experience”. (p. 2). These learning theories are not really learning styles, such as how one learns something; instead, they are more involved in how knowledge is acquired. There are many different ways to approach learning; the three main paths are behaviorism, cognitive constructivism and social constructivism. Different people have contributed to these theories. In the first learning theory, behaviorism, two theorists studied this question. They were John B. Watson and BF Skinner. The one who introduced behaviorism was John B. Watson. Watson defined behaviorism as "a natural science which takes over the whole domain of human adjustments." (p. 11 in behaviorism?). Now, BF Skinner thought the same thing as Watson about behaviorism. Watson and Skinner both argue that scientific theories should be observed as a stimulus-response rather than an observation from within the system.