Wilhelm Wundt is most commonly known as the `father of experimental psychology'. Having created the first laboratory of experimental psychology in Leipzig, Wundt, along with his assistant Oswald Kulpe, promoted the application of the experimental method in psychology. Ironically, from this incomplete perspective, Wundt's psychology could be viewed as being part of the natural sciences or naturwissenschaften and as such, in accordance with the necessity for empirical verification required by a positivist Machian philosophy.
Yet for Wundt, such an approach comprised only one half for the role of psychology as a science. Wundt distinguished between naturwissenschaften, in which psychology concerned itself with the associated physiology by means of experimentation, and geisteswissenschaften which loosely referred to understanding the individual as having subjective experience within a socio-cultural context, most commonly by means of introspection but also through an historical approach. It was this emphasis on psychology as being geisteswissenschaften and not solely a natural science, which meant it's unverifiable, metaphysical nature was incompatible with the positivist perspectives of Kulpe, Titchner and Ebinghaus.
In his works, Wundt outlined an important distinction between two types of causality; physical and psychic. The former refers to the relationship of the particular elements within a closed physical system, cogs in a machine for example. Crucially, mental phenomena such as subjective sensations are quite distinct, consisting of non-reducible `new values' and `new meanings' occurring in immediate experience. It is such phenomena that require psychic causality which cannot be investigated within a natural-scientific paradigm, because they are not subjectable to such physical laws of causality upon which the framework of natural science is structured.
For the positivists, knowledge (in any meaningful use of the term) was derived from the ability to verify observations in the form of data to provide empirical evidence for a proposed theory. If psychology were to be taken seriously it must therefore be possible to study all aspects both physiological and social by means of this method, or else not at all. Kulpe therefore rejected Wundt's notion of an `experiencing individual' as psychical with subjective processes (what we might now term `qualia') and instead conceived of the individual as a biological organism consisting of the physiological processes from which experience arose and thus could be experimentally investigated.
As well as the contrasting views between Wundt and the positivists on mental phenomena, its ontology and how it could be investigated, there were clearly two diverging views on what it meant for a subject to be a science. According to the philosophies of Avenaruis and Mach, by whom Kulpe and the later positivists were influenced, the key purpose of science was to generate `observations and descriptions' which could provide the `most economical summary of our experiences.' As such, there was a great emphasis on the unity of science with the various sciences being arrange in a hierarchical fashion, the most abstract, physics, at the top. At any point on the hierarchy, we should attempt to explain the observations and their functional relationships in the most economical way possible; that is, at the most abstract level. As such psychological phenomena should be reduced to physiological explanations.
This meant that the dualism Wundt placed on psychology as naturwissenschaften and as geisteswissenschaften would not adhere to the kind of positivist perspective requiring one (i.e. the latter) to be explained in terms of the other. In stark contrast with the positivists, Wundt argued that one set of sciences, for example those investigating subjective psychical causality, could not be explained in terms of the other, for example, those investigating the physics and physiology of psychology.
The tension seen between the emerging positivist paradigm and Wundt's emphasis on the apparent subjectivity of mental phenomena is somewhat analogous to that of contemporary debates with regards to philosophy of mind between, for example, Dennett and Chalmers. Such friction arises due to the required objectivity of the scientific method (in our case Popper's falsificationism, in Wundt's case, positivism) and the ontology of the subject matter being investigated. Based on the previous success of the current reductionist paradigm, scholars such as Dennett and Churchland attempt to apply such an approach to mental life purely because they believe it is the only way to advance scientific (and therefore meaningful) knowledge. Yet as pointed out by Chalmers, the cost of retaining the current methodological framework means explaining away mental phenomena in order to make it fit. In both cases, the ideas of Wundt and Chalmers were challenged because they did not allow compatibility between studying subjective mental phenomena within the prevailing paradigm.
Essentially, Wundt's ideas were incompatible with a positivist approach to psychology because the positivist need for verification of observable facts could not be reconciled with Wundt's claim for the apparent existence of subjective mental phenomena not open to such empiricism. One hundred and fifty years since Wudnt, four hundred years since Descartes, a vast array of technological, scientific and philosophical advances and the mind-body problem is as relevant as ever.