The idea of psychology as a science emerged during the middle of the nineteenth century as an intellectual result of the social influence of the historical achievements of natural sciences, and has ever been growing thereafter. In a sense, the modern history of psychology evolved mainly by the motive to promote psychology to the state of a science. However, at the time when the idea emerged, it was not clear as to both what science is and what psychology is. Thus, it was neither clear as to the theoretical contents and the categorical meaning of the idea of a scientific psychology. It was the actual historical pursuits of psychology as a science that provided us the theoretical contents to the idea of a scientific psychology, and the theoretical contents, in turn, cultivated the categorical meanings of the idea. Now, with the advantage of historical perspectives, we find that our understanding of the idea of a scientific psychology is closely related to, and reciprocally determined by, our understanding of the idea of science. The idea of science has its historical prototype in the natural sciences. Historically, there formed two different modes of understanding concerning the relationship between the idea of science and natural sciences as its historical prototype, thus determined two different ways of conceiving and realizing the idea of a scientific psychology, resulting in two different categorical meanings of the idea. With the actual history of modern psychology as a framework of reference, we might call the two different ways phenomenological and scientistic approaches to scientific psychology, respectively. The nature of the difference of the above mentioned two modes of understanding or ways of conceiving is not, however, of the kind in which the two modes or ways might be accepted indifferently as equally effective. Rather, they suggested and determined two historically opposite and logically antagonistic orientations for practicing and realizing the idea of psychology as a science. In the phenomenological approach, science means the logically necessary steps of the mental operations of consciousness, and the category of natural sciences becomes a special case of the mental operations in consciousness. This sense of science is not only logically necessary but also in harmony with the idea of psychology. So, only in the phenomenological sense of science can the idea of a scientific psychology realize itself. On the other hand, in the scientistic approach, science is identified with natural science. Historical analysis reveals that this sense of (natural) science can only be effective in the background of early-modern dualistic mode of thinking. In the dualistic mode of thinking, however, (natural) science established itself exclusively for the material world, while psychology established itself exclusively for the spiritual world. Thus psychology and (natural) science formed two contrasting, if not antagonistic, kingdom of human knowledge. So, in the sense of (natural) science, the realization of the idea of a scientific psychology can only be a self-alienation for psychology proper.