 Τι kind of positions are in opposition to realism, what kind of positions can be deemed anti-realist in this sense. I'll start by a nice quotation, which I want us to discuss for a few minutes. The astronomer's job consists of the following. To gather together the history of the celestial movements by means of painstakingly and skidfully made observations. And then, since he cannot by any line of reasoning reach the true causes of these movements, to think up or construct whatever hypothesis he pleases, such that, on their assumptions, the self-same movements, past and future both can be calculated by means of the principles of geometry. It is not necessary that these hypotheses be true. They need not even be likely. This one thing suffices that the calculation to which they lead agree with the result of observation. That's another famous quotation, which appears in the anonymous preface of a famous book. The 1543 book by Nicolaus Copernicus, The Revolucionibus on the Revolution of the Celestial Spheres. It was initially taken to be written by Copernicus himself, who advocated the heliocentric system, that is, that the Sun is at the center of the solar system, and the planets, including the Earth, move about it. Which goes in opposition to the traditional and then dominant Ptolemac system of astronomy, according to which the Earth is at the center of the solar system, and the planets, including the Sun, move about the Earth. Given this preface, the thought was that Copernicus did not take his heliocentric system as being a true description of the solar system, but as a way to calculate the positions of the planets and the various other astronomical phenomena. It turns out that this preface was written actually by Lutheran priest Andreas Ossiander. Part of the reason for putting forward this anonymous preface was that he would like the book to avoid the censorship of the Catholic Church by its revolutionary conclusion that the Sun is at steel and the Earth moves about it. Be that as it may, this quotation that I presented has been taken to sum up a major conception of scientific theories, an instrumentalist view of scientific theories, that all that is required for a good use of scientific theories is that they are good instruments for prediction and control. They don't have to be true to be good. Scientific theories have to be just good in predicting various observable phenomena. Now, this was not Copernicus' view. He was more of a realist about his position, but this kind of view that scientific theories are useful instruments for prediction and control has become a major theme in philosophy of science and has been taken to be the key view behind instrumentalism. Now, instrumentalism has come in various varieties. One position that was discussed in the early 1920s was the so-called non-cognitivist instrumentalism. This was advocated among other people by Philip Frank, a member of the Vienna Circle. According to this position, the aim of science's prediction and theories are merely tools for this aim, in particular symbolic tools that do not aim to represent the real world. Theories are neither true nor false. They have only instrumental value that is cast in terms of predictions of future observations on the basis of present ones. I don't think anyone takes this view very seriously anymore, but back then it was very popular. The reasons that it became unpopular among philosophers is that on the one hand it offers a reconstruction of science which turns a perfectly meaningful practice where there is communication and understanding into a meaningless manipulation of symbols. On the other hand, it presents a totally causally disconnected image of the world. Given its way of reading the theoretical superstructure of theories, it's not even possible that the observable phenomena are connected to each other via processes and mechanisms that involve unobservable entities. A more popular variety of instrumentalism was developed by a rather famous philosopher and scientist of the beginning of the 20th century, Pierre Duem, in his rather famous and important book, The Amen Structure of Physical Theory, which was published in 1906 and it was soon translated into English. The view of Duem is that there is a sub-distinction between science and metaphysics. The claim is that science aims at description whereas explanation belongs constitutively to metaphysics. Driven by his opposition to atomism and his defense of the phenomenological energetics, Duem envisaged the autonomy of physics which was seen by and large as dependent on a strict conception of scientific method captured by the slogan, scientific method is just experience plus logic. So for Duem, science is concerned only with experience and as such, as he says, it's not an explanation, it is a system of mathematical propositions deduced from a small number of principles which aim to represent as simply, as completely and as exactly as possible, a set of experimental laws. However, Duem himself pointed to a great difficulty that this instrumentalist view of science is supposed to face. He offered one of the best arguments against instrumentalist conception of scientific theories. And this argument goes like this, that if the instrumentalist story is correct, if theories are just tools for prediction and control, then it's not easy to explain how science entails novel predictions. If a theory were just a act filled with tools as Duem put it, it would be hard to understand how it can be a profit for us, how that is a predict phenomena which were hitherto unforeseen, unknown and predicted mostly by theories. The fact that some theories generate novel predictions could not be accounted for on a purely instrumentalist understanding of scientific theories. For how can one expect that an arbitrary classification of a set of non-experimental laws that is a classification that is based solely on considerations of convenience will possibly be able to reveal unforeseen phenomena in the world. Barring persistent coincidences an adequate account of the fact that a theory generates novel predictions can only rest on the claim that the theory has somehow latched onto the world, that is the claim that its principles and hypotheses correctly describe the mechanisms of processes which generate this phenomenon. Finally, a position which is akin to instrumentalism and I want to discuss is called Reductive Empiricism and was developed by Rudolf Karnab in the late 1920s, early 1930s. According to Karnab, all theoretical assertions can be translated into observational assertions without residue. That is the full content of a scientific theory is captured by what they say about the observable world. This view of translatability or reducibility of theoretical discourse into observational one came to grief in the late 1930s when Karnab himself understood that theoretical assertions assertions about electrons, protons, magnetic monopoles, DNA, molecules etc. cannot be given truth conditions in an ontology which dispenses with theoretical entities. If this is so, then a full explication of theoretical discourse simply requires commitment to reducible theoretical entities such as observational discourse requires commitments to observable entities. What empiricists came to call the excess content of theoretical discourse is captured by the fact that theoretical discourse is about unobservable entities.