Thirty years afterwards (1892) Wundt published a second, much abridged and entirely modified, edition of his work. The important principles of the first edition are entirely abandoned in the second, and the monistic is exchanged for a purely dualistic stand-point. Wundt himself says in the preface to the second edition that he has emancipated himself from the fundamental errors of the first, and that he "learned many years ago to consider the work a sin of his youth"; it "weighed on him as a kind of crime, from which he longed to free himself as soon as possible." In fact, the most important systems of psychology are completely opposed to each other in the two editions of Wundt's famous _Observations_. In the first edition he is purely monistic and materialistic, in the second edition purely dualistic and spiritualistic. In the one psychology is treated as a _physical_ science, on the same laws as the whole of physiology, of which it is only a part; thirty years afterwards he finds psychology to be a _spiritual_ science, with principles and objects entirely different from those of physical science. This conversion is most clearly expressed in his principle of psycho-physical parallelism, according to which "every psychic event has a corresponding physical change"; but the two are completely independent, and are not in any natural causal connection. This complete dualism of body and soul, of nature and mind, naturally gave the liveliest satisfaction to the prevailing school-philosophy, and was acclaimed by it as an important advance, especially seeing that it came from a distinguished scientist who had previously adhered to the opposite system of monism. As I myself continue, after more than forty years' study, in this "narrow" position, and have not been able to free myself from it in spite of all my efforts, I must naturally consider the "youthful sin" of the young physiologist Wundt to be a correct knowledge of nature, and energetically defend it against the antagonistic view of the old philosopher Wundt.