PHILOSPHY-PSYCHOLOGY

The relationship between logic and psychology was fought over most intensely in the German-speaking lands between 1890 and 1914. Indeed, during this period pretty much all of German-speaking philosophy was engulfed in the so-called Psychologismus-Streit (the ‘psychologism dispute’). This dispute centered on the question whether logic (and epistemology) are parts of psychology. Gottlob Frege and Edmund Husserl are the best-known figures of this controversy. The fact that the psychologism dispute has become closely associated with German-speaking philosophy must not, however, blind us to the enormous influence of John Stuart Mill upon both sides of the controversy. Paradoxically, Mill's Logic of 1843 was not only a key inspiration behind much German-speaking psychologistic philosophy, it also contained some crucially important anti-psychologistic ideas.

In what follows, I shall begin with a short summary of Mill's contribution. Subsequently, and turning to the German scene, I shall briefly sketch what authors accused of psychologism said on the relationship between logic and psychology, before outlining Frege's and Husserl's arguments against them. I shall conclude by presenting a number of objections to Frege's and Husserl's anti-psychologism. Some of these objections come from Frege's and Husserl's contemporaries, others are of more recent origin.

2. Mill's Psychologism

Critics and interpreters of Mill's philosophy of logic have been unable to reach a verdict on the question whether Mill was a psychologistic thinker. Recent work by David Godden (2005) provides a detailed explanation for this lack of agreement: Mill's position on the relationship between (deductive) logic and psychology is “fractured”. Some elements in Mill's thought push him towards a strongly psychologistic viewpoint, other elements pull him away from it. In other words, sometimes Mill insists that logic depends on psychology, sometimes he denies such dependence. (This section is based on Godden's important re-assessment.)

As Mill declares as the beginning of his System of Logic, logic has two parts: the ‘science of reasoning, as well as an art, founded on that science’ (1843, 4). The art of reasoning is prescriptive; it provides us with the rules according to which we ought to reason. The science of reasoning is a descriptive-explanatory psychological discipline and analyses mental processes. The overall goal of logic is ‘the guidance of one's own thoughts’ (1843, 6). The crucial question here is of course how the dependence of the art of reasoning upon the psychological science is to be understood. The mere fact that the art is prescriptive does not yet make it independent of psychology.

At this point it is important to note that Mill's science of reasoning really is, in both subject matter and methodology, a psychological discipline. Although the subject matter of the science of reasoning is more specific than that of psychology in general — the former investigates ‘the operations of the human understanding in the pursuit of truth’ (1843, 6) — there are, as Mill sees it, no special psychological laws characteristic exclusively of the domain of the science of reasoning. Instead, ‘the general laws of association prevail among these more intricate states of mind’ (1843, 856). Moreover, in both the general and the more specific domain, the discovery of laws is due to ‘the ordinary methods of experimental inquiry’ (1843, 853).

Mill offers at least three different accounts of the contribution which the psychological science of reasoning makes to the art of reasoning. On the first account, the prescriptive art of reasoning seems prima facie independent of psychological science. In this vein Mill writes, for example, that psychology can only offer ‘the analysis of the mental process which takes place whenever we reason’, whereas the art of reasoning must provide ‘the rules … for conducting the [reasoning] process correctly’ (1843, 4). A second account is based on the idea that any successful prescription for a given type of thought process must be based on a proper and detailed psychological understanding of that process: ‘A right understanding of the mental process itself … is the only basis on which a system of rules, fitted for the direction of the process, can possibly be founded’ (ibid.). And Mill's third account goes further still: ‘Its [the Science of Logic's] theoretic grounds are wholly borrowed from Psychology, and include as much of that science as is required to justify the rules of the art’ (1865, 359). On the third account, psychology is essential to the justification of the rules of reasoning. Mill provides no clear indications on how these prima facie conflicting statements can be reconciled. Clearly the third statement will qualifies as psychologistic by everybody's criteria — though doubts have been raised whether, ‘when this passage is read in its full dialectical context’, it amounts to more than the claim that ‘the logician must formulate rules of inquiry in a manner which will be as helpful as possible to inquirers, and must draw on the psychology of thought to do so’ (Skorupski 1989, 166). But even the first two accounts, interpreted against the backdrop of Mill's other views, make prescriptive logic dependent upon psychology. Logic cannot begin its task until psychology has provided a descriptive analysis of reasoning processes.

We encounter a similar mixture of psychologistic and anti-psychologistic motifs also as we turn to Mill's views on how the logician ought to justify logical precepts and principles. For Mill, we specify the rules of deductive inference by identifying the valid forms of the syllogism (1843, 168). And validity is explained by the formula ‘if the premises are true, the conclusion must inevitably be so’ (1843, 166). ‘Good inferences’ have an connection to truth: ‘A reasoning, to be rightly framed, must conduct to a true conclusion’ (1865, 365). And hence

… at bottom the only important quality of a thought being its truth, the laws or precepts provided for the guidance of thought must surely have for their principal purpose that the products of thinking shall be true (1865, 365).

A ‘good’ inference is thus a ‘true inference’, an inference ‘grounded in the reality of things’ (1843, 10–11).

In line with this consideration, Mill distinguishes between act, content and object of a judgement. The content of a judgement accounts for its representational connection to the external world. And the truth of a judgement depends on its object, that is, the state of affairs represented by the content. Thus when it comes to identifying valid inferences, logic must focus on objects (1843, 87). Mill's theory of judgements and their truth involves a rejection of ‘conceptualism’, the view that the objects of judgements are mental, and that ‘a proposition is the expression of a relation between two ideas’ (1843, 109). Against this view Mill insists that ‘propositions … are not assertions respecting our ideas of things, but assertions respecting the things themselves’ (1843, 88).

It is easy to understand why some authors have taken such statements as evidence that Mill was defending an anti-psychologistic position of sorts. And yet, we must remember that these statements directly contradict other of his pronouncements, some of which were quoted earlier. Recall only that Mills defines logic as ‘the science of the operations of the human understanding in the pursuit of truth’ (1843, 6). There is no denying that Mill's theory of logic contains incompatible views.

Godden (2005) shows convincingly that even Mill's object-focused project eventually circles back towards psychologism. First of all, Mill offers affirmative and negative formulations of the Principle of the Transitivity of Co-existence as the ultimate foundation of all syllogistic reasoning:

The first, which is the principle of affirmative syllogism, is, that things which co-exist with the same thing, co-exist with one another … The second is the principle of negative syllogisms, and is to this effect: that a thing which co-exists with another thing, with which other a third thing does not co-exist, is not co-existent with that third thing (1843, 178).

Second, although Mill never seeks in turn to provide a justification for the Principle of Transitivity of Co-existence, it is not difficult to work out what for him such justification would have to look like. The parallel case of geometric axioms is clear enough: ‘It remains to inquire, what is the ground for belief in axioms — what is the evidence on which they rest? I answer, they are experimental truths; generalisations from observation’ (1843, 231). For Mill all general propositions must ultimately be justified by experience:

And so, in all cases, the general propositions, whether called definitions, axioms or laws of nature, which we lay down at the beginning of our reasonings, are merely abridged statements, in a kind of shorthand, of the particular facts, which, as occasion arises, we either think we may proceed on as proved, or intend to assume (1843, 192).

Third, Mill reverts to a full-blown form of psychologism when turning to the justification of other logical principles, in particular the Principle of Non-Contradiction. Mill insists that it too is an empirical generalisation over our experience, and especially over our inner experience: