This paper was published in the South African Journal of Philosophy, Vol. 8 No.1, pp.1-7. Feb.,1989.
Philosophical Method
Republic of South Africa
abstract
In this article the author argues that the method of conjecture and refutation proposed by Popper for science is, as a method, effective and sufficient for philosophy. The method is effective in those fields of philosophy where truth is fundamental to the first order theories, and to the philosophical theory, such as epistemology, philosophy of science, philosophy of language, philosophy of psychology, etc. If this is true then most of what Rorty says about method and the nature of philosophy in Philosophy and the mirror of nature is false. Sceptics such as Wittgenstein are also refuted.
abstract in Afrikaans
In heirdie artikel betoog die outeur dat die metode van gissing en weerlegging wat deur Popper vir die wetenskappe voorgestel is, ’n doeltreffende en afdoende metode vir die filosofie is. Die metode is doeltreffend op daardie filosofiese terreine waar waarheid fundamenteel is vir die eerste-orde-teoriee en vir die gepaardgaande filosfieses teoretisering soos kennisleer, wetenskapsfilosofie, taalfilosofie, filosofie van die sielkunde en so meer. As hierdie konklusie geldig is, dan is die meeste wat Rorty oor metode en die aard van die filosofie te sê het in Philosophy and the mirror of nature, vals. Skeptici soos Wittgenstein is dan ook weerlê.
In my view philosophers should be concerned with philosophical problems, and with ‘progressive’ philosophical research programmes. To be able to satisfy this requirement they need a method, which is effective for producing a set of non-false theories, hypotheses and basic statements. Is there such a method? I argue that such a method exists. It is the method of conjecture and refutation as proposed by Popper for science.
1. How the problem arises
Is there a problem about method in philosophy? How does it arise? Firstly, one can frame questions such as, Is there is a method in philosophy?, Are the philosophers who claim there is no method peculiar to philosophy (Popper, Feyerabend, Wittgenstein) saying something which could be true? Are the philosophers who say that philosophy differs only in degree from the sciences, (Carnap, Quine) and hence that its methods must be scientific, making a defensible claim? When I attend a philosophy department staff seminar which method ought I to expect to see followed in the pursuit of philosophical truth? Or for those philosophers for whom truth is not the point, what method is or should be normative for the defence of a philosophical position?
Secondly, one wonders what method is being followed in some of the papers one hears at philosophy conferences, staff seminars, spring colloquia etc. One wonders whether the author has any idea of how he is going to establish the truth of or justifiability of his claims, especially in those cases where the language used is so opaque that even the attentive auditor cannot understand what is being asserted. Einstein said different things about philosophy at different time. Once he delivered himself of the following: ‘Is not all of philosophy as if written in honey? It looks wonderful when one contemplates it, but when one looks again it is all gone. Only mush remains.’ (Rosenthal-Schneider 1980:622). The charitable thing to do is to imagine that in talking about philosophy Einstein was talking about sort of philosophy prevalent in Switzerland and Germany between 1890 and 1905, namely, Hegelian absolutism, the post-Kantian rationalism of Schopenhauer, or the so-called ‘European Nihilism’ of Nietsche.²
Thirdly, Richard Rorty has attacked Anglo American Analytical Philosophy in Philosophy and the mirror of nature (1980) and claimed that there is no method in analytical philosophy sufficient for producing true theories or statements about philosophical knowledge or the world of science. Philosophy in his view cannot do what the great tradition of Plato, Descartes and Kant claimed it could, namely, be the Queen of Sciences or produce philosopher kings, or generate privileged knowledge or insights about nature, morality or man. I do not think Rorty’s claimed will go away if we ignore them, or scoff at them, or address ad hominem remarks against Rorty. European intellectuals are beginning to become familiar with Rorty’s work and to take him seriously.
I tried to refute his claims in my inaugural lecture, ‘Is the end in sight for philosophical knowledge’ (1981) but I admit that I was not able to produce a method at that time which would have been adequate to counter Rorty’s scepticism about philosophy as the Queen of the Sciences. I denied and argued against all his major claims and referred to the methods used by philosophers like Quine in arriving at the philosophical position which had not been shown to be false. Although these methods could not support the grand design of philosophy as an intellectual overseer of the sciences and humanities, they could, I argued, (1981) support the view of philosophy as the handmaiden of the sciences and humanities.
It will be useful to remind ourselves of some of the details of Rorty’s attack on Anglo American Analytic philosophy. The concise statement of his view I give here is similar to the one I gave in 'Is the end in sight for philosophical knowledge?' (1981). Rorty (1980:129ff) argues that our philosophical tradition, which he calls ‘the great tradition’ originated with the Greeks and was amended by Descartes and Kant. Although this tradition can be analysed into many separate theses, Rorty's generally realistic view of truth and knowledge is one which conceives of truth as some form of correspondence between thought and nature. Knowledge is generally thought of as consisting in the possession of ideas or concepts or representations which accurately mirror nature. Rorty attributes this idea to the Greek philosophers, particularly Plato. He therefore thinks of this tradition as thinking of man as a king of mirror of nature.
Rorty claims that the second principal strand of modern philosophy was contributed when Descartes invited us to think of the mind as a logically private interior stage on which cognitive dramas are enacted. Knowledge now becomes an interior representation of objective reality and the mirror becomes an inner mirror. Rorty thinks that Descartes bequeathed to us the problematic concept of consciousness, or the so-called mind/body problem. In the American Philosophical Association Symposium in December 1980, Kim (1980:589) pointed out that the ‘mental turn’ taken by Descartes led to discussions of that which could be ‘incorrigibly known’, necessarily also gave rise to sceptical queries about whether there was any reason for thinking that there was an outer mirror.
The third thesis was contributed by Kant who thought that it was the task of philosophy to examine the foundations of the sciences and humanities and of the moral life. Philosophy was required to produce universal standards of rational judgement not only for actual claims of knowledge but for all future claims. There is a sense here in which the philosopher is a kind of cognitive adjudicator and overseer, concerned with the various presuppositions and methods of the different sciences and humanities. Rorty alleges that these three basic conceptions of knowledge and philosophy define the Western philosophical and Continental philosophy, with both branches as loyal heirs to the dominant image of man as a mirror of nature.
Why does Rorty want us to reject this conception of philosophy? First Rorty argues that this traditional conception of philosophy presupposes that we can engage in the sort of foundationalism, which incorporates both the construction of foundations for knowledge and the evaluation of the foundations presupposed by non-philosophical subjects, and it assumes that a rational and objective method is to hand for reaching agreement about truth and legitimation. Rorty thinks that Kuhn’s work in the philosophy of science has shown that this kind of rationality and objectivity is a cultural myth which is not available to philosophers or, indeed, anyone else. If so, philosophers cannot be required to perform an impossible task. Secondly, Rorty believes that the idea of ‘correspondence’ is at the base of the doctrines of Platonic and Kantian realism which form the orthodoxy in the Western philosophical tradition. Rorty thinks that it is not possible to give a clear and and non-metaphysical account of the idea of ‘correspondence’. Rorty (1980:311) writes ‘The trouble with Platonic notions is not that they are “wrong” but that there is not a great deal to be said about them – specifically there is no way to “naturalize” them or otherwise connect them to the rest of enquiry or culture or life.’ Thirdly, Rorty thinks that there is not any way available to us in which we can present arguments on whether to impose a Kantian type of ‘grid’ on experience or to set it aside in favour of some other successor subject. He thinks this because he believes that there is no ‘normal’ philosophical discourse or standard neutral philosophical paradigm which could provide a common ground between the sciences and the humanities or between the different philosophical paradigms and schools which exists in 20th century philosophy.
In general the Great Tradition sees philosophy as a meta-science. The Queen of the sciences must herself be a type of science; otherwise she could not be a guardian of knowledge, objectivity, rationality and truth. However, the Platonic-Cartesian-Kantian grid cannot do what is required of it because of the nature of its generalizations. For example, it can produce a theory of truth as correspondence between sentences and nature, but the very generality of the theory will prevent it from having any direct bearing on a specifiable scientific problem. Philosophical generalizations about truth will thus not be falsifiable in the same way as scientific generalizations are, and hence philosophy cannot act as a truth claim adjudicator in any one particular science. By a similar argument, philosophy cannot be a cognitive claim adjudicator in the realm of first order knowledge claims. So Rorty thinks that the Platonic-Cartesian-Kantian grid is theoretically incapable of carrying within itself the commensurating ground required for the fulfilment of its Queenly role. It is not only the Great Tradition which fails to be an objective, rational ground, but all of its 20th century descendants inherit the illusion of being a rational, objective problem-solving activity. I shall be content with the foregoing as an account of what Rorty thinks the great tradition is and why he wishes to reject it.
Fourthly, other philosophers have been sceptical about philosophical method. Notably Popper and Wittgenstein. In the preface to the 1959 edition of The logic of scientific discovery Popper (1959:15) says, ‘Philosophers are as free as others to use any method in searching for truth. There is no method in searching for truth. There is no method peculiar to philosophy.’ He goes on to say, ‘And yet, I am quite ready to admit that there is a method which might be described as “the one method of philosophy”. But it is not characteristic of philosophy alone; it is, rather, the one method of all rational discussion, and therefore of the natural sciences as well as of philosophy. The method I have in mind is that of stating one’s problem clearly and of examining its various proposed solutions critically.’
Wittgenstein thought there could not be genuine philosophical problems, and hence no genuine philosophical method. His remarks are scattered through various writings: The claims in the Philosophical investigations are already anticipated in the Tractatus (1922) where the correct method for philosophy is described:
6.35. The right method of philosophy would be this. To say nothing except what can be said, i.e. the propositions of natural science, i.e. something which has nothing to do with philosophy: and then always, when someone else wished to say something metaphysical, to demonstrate to him that he had given no meaning to certain signs in his propositions. This method would be unsatisfying to the other – he would not have the feeling that we were teaching him philosophy – but it would be the only strictly correct method.
6.54. My propositions are elucidatory in this way he who would understand me finally recognises them as senseless when he has climbed through them, on them, over them. (He must so to speak throw away the ladder, after he has climbed up on it.) He must surmount these propositions; then he sees the world rightly.
7. Whereof one cannot speak, thereof one must be silent.
Wittgenstein did not follow his own advice in the Tractatus, instead there are plenty of philosophical propositions. However these paragraphs prefigure the pathological/prophylaxis view of philosophy was a kind oh therapy, because ‘doing philosophy’ was able to cure philosophers of the desire to do philosophy. Instead of discarding the ladder you can discard the therapy, if therapy works.
For the purposes of this article I shall take the foregoing as an adequate introduction to my views of how the problems connected with the idea of a correct philosophical method may be generated.
2. Conjecture and refutation in science
In my view those philosophers and others who claim that there is no method in philosophy are wrong. There is a method which is effective and suited to philosophy. It is the Popperian method of conjecture and refutation. The method is deductivist. It will also count as a rationalist method, although it is not rationalist in the traditional sense. In this article I shall claim that the method works effectively in those fields of philosophy where truth values and truth conditions are fundamental, such as epistemology, philosophy of science, philosophy of language, philosophy of psychology, etc. I shall not in this article claim that the method is straightforwardly applicable to ethics and social philosophy.
Here an objection may be raised. The objector might say that my proposal is in effect Popper’s idea. I concede that my proposal consists in applying Popper’s idea of the correct method for science to philosophy. However, it is not true that Popper believed that this method was one in which one could have confidence when applied to philosophy. I support this claim by reference to his thesis mentioned in 1 above that there is no method peculiar to philosophy; and to section 2 of Chapter 8 of Conjectures and refutations, titled ‘The problem of the irrefutability of philosophical theories’ in which he argues that philosophical theories are irrefutable. He discusses five examples of apparently irrefutable theories, which he believes to be false! He (1963:197) then asks: ‘If philosophical theories are all irrefutable, how can we ever distinguish between true and false philosophical theories? This is the serious problem which arises from the irrefutability of philosophical theories.’
I shall return to this question later, but for the moment it is sufficient to note that some philosophical theories may be irrefutable, and that Popper himself has never pressed for the method he applies to science to be applied to philosophy. Nor, to the best of my knowledge, have any of the students of Popper. However, Lakatos has successfully applied the method to proof procedures in mathematics, and Ernst Gombrich had tried to apply the method to art and, aesthetic objects in Art and Illusion (1959). I cannot say whether he has been successful, I can only say that it is and extremely thoughtful and thought-provoking book.
I turn now to the hypothetico-deductive method which Popper had argued is the only adequate method for science. The proposals have of course been modified by and in ht debates on falsification on fallibilism since 1970. The method arises out of Popper’s attempt to solve the problem of induction, which has been called the skeleton in the cupboard of modern philosophy. Popper notes the asymmetry between the conditions of truth and falsity of a universal generalization of the form (Ax) (Fx -> Gx). If the domain is potentially infinite, as it would be if the variables ranged over the electrons or neutrinos, then no number of confirming instances would logically imply the truth of the generalization. However, one single counter-example would be sufficient to falsify the generalization. Suppose we claim for any electron its mass is 9.11 x 10^-31 kg, then even if we had examined 10 billion electrons and found that they conformed to the mass specification, that would not imply the truth of the generalization, (Ax) (Electron x -> the mass of x = 9.11x 10^-31kg.) But a single electron with mass 2 x 10^-22 would falsify the generalization. This is Hume’s problem. Popper takes Hume seriously. But Poper has a solution different to that proposed by Hume. In Popper’s theory ‘refute’ is, with lots of qualifications, going to stand in the same relation to a subset of scientific theories of hypotheses, as ‘falsify’ does our generalization if one electron fails to meet the mass specification.
A naïve dogmatic falsification (a type of unintelligent straw-man invented by Lakatos) believes in crucial experiments, that the only acceptable counter-evidence to a theory is empirical evidence, and draws a sharp distinction between the theoretical and the experimental. So when an experiment is both crucial and empirical, the naïve falsification insists that the theory be abandoned forthwith.
The sophisticated methodological falsificationist separates rejection from refutations, and feels free to hang on to an apparently refuted theory, by use of ad hoc hypotheses, or by attacking the theoretical basis of the experiment which allegedly provided the refutation, Popper and the sophisticated falsificationist thinks of the growth of science as partly dependent on the policy of no counting a particular theory as falsified by an agreed upon observation, or basic statement, or experiment, unless there is a rival theory in which the observation statement or experiment can be turned into a successful prediction. I shall give an example. Newton’s theory has enormous explanatory power, and is able to generate extremely accurate calculations, especially in the realm of terrestrial quantification. And although many had tried no falsifications of Newton’s theory were accepted from 1697 until 1905 when Einstein published his paper on special relativity.³ And it said that there were only twelve people in Europe who would in 1905 understand the import of what Einstein had done, as opposed to the content of his paper on special relativity. The import was the falsification of Newton. This is important. A theory may have been falsified without many people realizing it.
Newton’s theory implies that the planets move through aether, and that the perihelion of Mercury (the point at which it is closer to the sun) moves 43 arc-seconds per 100 years. The Michelson, Morley experiments showed that if there were an aether through which the earth moved the movement of the earth would be slowed down by it, and that the aether, but not Newtonian theory.
Astronomical observations showed that the orbit of Mercury moved 43 arc-seconds per century, which was taken to be an anomaly for Newton, but not something to be bothered about. Einstein was bothered about Mercury’s perihelion. He took the Michelson, Morley result and the anomalous orbital calculation to be a falsification of Newton’s theory.
The Michelson, Morley result was not that important for Einstein for he had already developed a theory which contained two new postulates which helped to explain certain difficulties which some theorists had in accepting the Michelson, Morley result. The postulates were: (1) The laws of physics take the same form in all inertial frames. (2) In any given inertial frame, the velocity of light c is the same whether the light is emitted by a body at rest or by a body in uniform motion.
If the only falsification to Newton’s dynamics, statistical mechanics, and gravitation theory were the Michelson, Morley results and the perihelion anomaly, then Newton’s rule of 208 years would have continued. But Einstein showed in addition that there was an alternative theory of gravitation, space-time and kinematics, which could explain all that Newton could, which did not assume the existence of aether, and which implied that the perihelion of Mercury moved 43 arc-seconds per 100 years (Pais 1982:11-176).
In Popperian method if a theory survives the ordeal of severe tests of that which it predicts and forbids then it is said to be ‘corroborated’. The more severely it is tested the more highly it is corroborated, and is hen said to have ‘verisimilitude’. Although corroboration is not the criterion of verisimilitude, it is an indication of it. All attempts at falsification which do not succeed become corroborations, and so increase our belief in the degree of verisimilitude of the theory. On Popper’s account growth occur when one rejects a theory in favour of a rival theory, because the rival theory has greater empirical content, and is more corroborated then the rival. And since Popper argues that corroboration is an indication of truth-likeness or verisimilitude, the rival theory has a higher degree of verisimilitude.
For Popper, all the as yet unfalsified basic statements, hypotheses, and theories, are part of public objective knowledge and are members of the Third World.
Also important is the demarcation of those theories, hypotheses and ideologies which are not amenable to even potential falsification. This is the realm of non-science, and these objects are not members of the Third World.
3. Conjecture and refutation in philosophy
For brevity let us call the method outlined in 2 above PM. Let us suppose that the hypothesis:
(a) that PM is the only correct method in philosophy for generating the non-false, well-corroborated, high verisimilitude philosophical theories/hypotheses/basic statements
is it well corroborated.
What should we expect? Since PM has both a normative and descriptive implication, we should expect that the implicative force of (a) would include both modes. We should expect that (a) would apply in both normative and descriptive contexts. We should further expect that some of the implications of the non-falsity of (a) would be that:
(i) The hypothetico-deductivism of PM would characterize the context of formalization of a philosophical theory/hypothesis/basic statement. The content of formalization is the same as the context of justification, except that it is stripped of any inductivist, or empiricist assumptions. The strategies of PM and the implications which we may derive from it about how to proceed in a problem situation would also apply to the context of discovery.
(ii) we should expect in either the context of formalization or of discovery of a theory of basic statement, that a great deal of time is devoted to attempted refutations and falsifications. Staff seminars, spring colloquia, international conferences would and should be characterised by numerous and repeated attempts and falsifications.
(iii) we would not expect to find philosophers resorting to hundreds of cases of induction so as to be able to form one or more generalizations based upon them. Of course there may be some who are a bit confused about method in philosophy, and they might be doing just that. We ought instead to find philosophers in their chairs framing hypotheses and theories without resorting to induction as a necessary part of philosophical method.
Philosophers would be free to use a principle analogous to the principle of mathematical induction, namely, if M is a set of formulas, and P is a certain property of formulas (e.g. the truth value t) then to show that P holds for all members of M it is sufficient that two conditions are satisfied: (1) Every element of M of degree zero has P; (2) If some element of M of degree > 0 fails to have P, then some element of M of a lower degree fails to have P. The virtue of a principle analogous to mathematical induction is that philosophical reasoning can be carried out without any empiricist appeals.
(iv) one formal expectation from this theoretical point of view would be: inductivism as the method of philosophy in the context of formalization would be refuted and rejected.
(v) the method of transcendental deduction of Kant and the post-Kantians would be discarded. It would be replaced by (a).
(vi) the claims of the ordinary language philosophers or linguistic analysts that there are no real problems in philosophy, and hence that there is no method except the method of analysing ordinary discourse would be refuted and rejected and replaced by another form of rationalism, namely, that implicit in (a). This began to happen in Oxford and Cambridge towards the end of the 1960s. In the sixties more Oxford graduate students chose to write theses on topics compatible with the framework and method of linguistic philosophy, but by the early seventies it was cause of concern at Oxford that almost all new research degree theses were on problems originating from the works of more science oriented American philosophers, who openly derided Wittgenstein and the linguistic school of philosophy.
(vii) the history of philosophy would be strewn with the corpses of refuted and rejected theories. It would also be extremely unlikely that theories from the ancient of medieval philosophers would survive unfalsified over such a long time gap.
(viii) another formal requirement would be that any theory of philosophical method incompatible with PM would have to be refuted, if not immediately, then over time.
(ix) Rorty would be refuted. Philosophy would, as in the case of science, have a method which would be effective and efficient for producing philosophical theories/hypotheses/basic statements. Philosophy would no longer be the Queen of the Sciences, or the cultural managing director of the humanities. It was never capable of any of those grand, but impossible roles. Why? Because the correct method for philosophy has always been PM, and since anyone using PM in either science or philosophy is an epistemological equal, no theorist of knowledge can claim to more exalted, or grander than any other. However, philosophy can and has made a distinctive contribution to the Third World because of the vast scope of philosophical generalizations in epistemology, the philosophy of science, and elsewhere.
(x) Popper would be refuted on the question of whether all philosophical theories are irrefutable. Contra Popper some philosophical theories would be refutable in principle and in fact. And many potentially falsifiable ones would be members of the Third World.
(xi) we should be able to demarcate philosophy from non-philosophy. Many of the, in principle, unfalsifiable theories/hypotheses/basic statements would not count as philosophy, or be members of the Third World, although some of the more metaphysical claims would. We would have a clear distinction between the philosophical theories which were refutable and those that were not. We would have to take a decision to allow some unfalsifiable hypotheses to be members of the Third World, at least the scientific and philosophical part of the Third World, would be uncontroversially established. There would be none of the difficulties which attend inductivist attempts to say which theories and basic statements we can trust.
4. The refutation of philosophical theories
In this section I am going to claim that a number of theories in philosophy either have been falsified or that we know what would falsify some as yet unfalsified theories.
Davidson’s theory of anomalous monism
According to Davidson (1980:208) the theory of anomalous monism implies that all events are physical, and presupposes the falsity of the materialist claim that mental events can be given purely physical explanations. The theory allows that (i) possibly not all events are physical, are both true. This depends on Davidson’s successful way of reconciling three apparently inconsistent principles: (a) at least come mental events; (b) where there is causality there must be a law; and, (c) there are no strict deterministic laws on the basis of which mental events can be predicted and explained. The anomalous monist cannot be a reductionist because of (1) the holism of the mental, (2) the anomalous of the mental descriptions, which are not to be jettisoned, and (3) the theoretically expected absence and impossibility of psycho-physical laws. Davidson’s theory would be falsified if it turned out that (c) is false, and that it could be demonstrated that most or all mental predicates or mental terms could, in some rival theory be reduced to physical terms.
Parfit’s theory of personal identity.
The theory requires that mental states are logically distinct from the particular body with which they happen to be associated at time t. If we could show that for be to show that for any genuine philosophical theory or hypothesis we can know what would falsify it. And to be able to isolate those philosophical theories that we want to take seriously and regard as genuine, but which are in principle irrefutable.
I have shown above that philosophical theories/hypotheses/basic statements are falsified or refuted in various different ways. The most obvious is by counter-examples. Examples are either drawn from actual world cases of from possible worlds. I submit that this method occurs frequently in philosophy department staff seminars, and at conferences.
Another type of refutation is to show tat the proposed theory or hypothesis is incompatible with unproblematic current scientific knowledge. For example Aristotle’s idea, subsequently amended in various ways by various historical figures, of the nature of substance and accident is incompatible with our knowledge of micro-physics and the quantum mechanical descriptions of objects.
A theory or hypothesis could be incompatible with general logic principles, or general philosophical principles, which we have no reason to suspect as problematic. Sometimes we say that a theory is incompatible with our ‘intuitions’. We have to be careful here. If the alleged ‘intuitions’ come from the realm of common sense, there may be no reason to trust them. If however they come from well-corroborated theories or basic statements that is different. But in this latter case we would not need to call them intuitions, we could simply refer to reject or refute a theory, especially if an alternative theory is to hand which does not conflict with our ‘intuitions’ but which accounts for all the cases which the disputed theory does. I think that each case has to be examined to see what sort of ‘intuition’ is being appealed to, and to see whether we cannot appeal to background knowledge, or some other as yet unfalsified hypothesis to do the work which the intuition was called upon to do.
In order to establish PM as the sole method of philosophy a lot more would be required than I have been able to show above. One would have to tackle the question of how the method could apply to fields of philosophy where truth conditions and truth values apparently not fundamental, such as in ethics, and political philosophy.
1. For the meaning of ‘progressive’ see I. Lakatos, 1970, p.116ff.
2. The evidence (cf. Pais 1982:318-319) shows, however, that Einstein had read and was familiar with Kant whilst a schoolboy, and in later years had read Spinoza’s Ethics, Hume’s Treatise of knowledge, and some Plato.
3. Einstein, Annalen der Physik, 1905 the first relativity paper is in vol. 17 p.891 and the second which contains the equation E = mc² is in vol.17, 9.639. For example, the rejection of the corpuscular theory of light in favour of the wave theory was not viewed as a falsification.
Davidson, D. 1980. Mental events, in D.Davidson, Essays on actions and events. Oxford: Oxford U.P.
Gombrich, E.H. 1959. Art and illusion. London: Phaidon.
Kim, J. 1980. Rorty on the possibility of philosophy. Journal of Philosophy, 77(10).
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Wittgenstein, L. 1922. Tractatus Logico Philosophicus. London