Okasha, S. (2007). What does Goodman's 'Grue' Problem Really Show? re-reading of Frank Jackson, 'Grue', Journal of Philosophy 5 (1975), pp.113-131. Philosophical Papers, 36(3), 483-502. https://doi.org/10.1080/05568640709485211 Peer reviewed version Link to published version (if available): 10.1080/05568640709485211 Link to publication record in Explore Bristol Research PDF-document This is an author's accepted manuscript of an article published in Philosophical Papers, Volume 36, Issue 3, 2007, copyright Taylor & Francis, available online at: http://www.tandfonline.com/10.1080/05568640709485211 University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ Philosophical Papers Vol. 36, N o. 3 (N ovem ber 2007): 483-502 W hat Does Goodm an’s ‘Grue’ Problem Really Show? Samir Okasha Re-reading of Frank Jackson, ‘Grue’, Journal of Philosophy 5 (1975), pp.113-131.∗ Introduction N elson Goodm an devised his fam ous ‘grue’ exam ple in order to show that the sim ple ‘straight rule’ of induction leads to inconsistency unless restricted to the so-called ‘projectible’ predicates. In his searching critique of Goodm an’s discussion, Frank Jackson (1975) argued that on careful exam ination, Goodm an’s alleged paradox dissolves—the naïve straight rule does not lead to inconsistency, and does not need to be restricted to predicates of any particular type. Jackson’s article seem s to have been overlooked by m uch of the subsequent literature on ‘grue’; indicative of this is that virtually all philosophers today believe that Goodm an did show what, according to Jackson, he did not show.1 The structure of this paper is as follows. Section 1 briefly rehearses Goodm an’s m ain argum ents regarding ‘grue’, to set the stage for Jackson’s critique. Sections 2 to 5 expound Jackson’s argum ent, com m enting on a num ber of points. Section 6 discusses the consequences of Jackson’s position, and offers som e suggestions about ∗ Editor’s Note: ‘Re-Readings’ is a regular feature in Philosophical Papers. Authors are invited to write on a past article, book, or book chapter that they deem , for whatever reason, to deserve renewed attention. Authors are encouraged, where appropriate, to discuss the work’s reception by and influence upon the philosophical com m unity. 1 Though an im portant recent exception is P. Godfrey-Sm ith, ‘Goodm an’s Problem and Scientific M ethodology’, Journal of Philosophy 2003, pp. 573-590; see the discussion in Section 6 below. 484 Re-Reading why Goodm an’s discussion of ‘grue’ has so frequently been thought to contain a deeper philosophical m oral than in fact it does. 1. The Straight Rule of Induction In Fact, Fiction and Forecast, N elson Goodm an argued that the project of trying to justify inductive reasoning—the ‘old problem of induction’— was fundam entally m isguided, and should be replaced by the project of trying to describe inductive reasoning, i.e., of specifying which inductive inferences we regard as rational and which not. The natural starting point for this m ore m odest descriptive project, Goodm an argued, is the so-called ‘straight rule’ of induction, i.e., arguing from ‘som e Fs are G’ to ‘all Fs are G’ or ‘the next F will be G’. (Following Jackson, I use ‘SR’ as an abbreviation for ‘straight rule’.) Since Hum e, this argum ent pattern has often been regarded as the basic schem a for inductive inference, covering m any of the actual inferences we m ake in everyday life. However, Goodm an argued that the straight rule, in its unqualified version above, provides a very im perfect description of real-life inductive practice, and in fact leads to paradox. He called this the ‘new riddle of induction’. The paradox arises, Goodm an claim s, because from equivalent evidential bases, the straight rule can lead us to incom patible predictions. M ore precisely, from the logically equivalent statem ents ‘all observed em eralds are green’ and ‘all observed em eralds are grue’— where ‘grue’ is defined as ‘green and observed or blue and unobserved’—the straight-rule leads to the predictions ‘the next em erald to be observed will be green’, and ‘the next em erald to be observed will be grue’, which are incom patible. U nrestricted application of the straight rule therefore leads to inconsistency. Goodm an’s own solution to this supposed paradox was to restrict the class of predicates to which the straight-rule can be applied. Predicates such as ‘green’ are projectible, he argued, while ones such as ‘grue’ are unprojectible. The problem of describing our inductive reasoning—at least in so far as it follows the ‘straight rule’ pattern— Re-Reading 485 thus becom es the problem of distinguishing the projectible from the unprojectible predicates. As Jackson notes, the extensional aspect of this problem has usually been regarded as straightforward: m ost philosophers, following Goodm an, have assum ed that we know which predicates belong in the projectible class and which not. However, there has been widespread disagreem ent over the intension of this class, i.e., over what makes a predicate projectible. Goodm an’s own solution— that ‘entrenchm ent’ is what m akes a predicate projectible—has m et with widespread criticism , and alternative theories of projectibility abound.2 O n two points there is near universal consensus, however: firstly that there is a projectible/non-projectible distinction, and secondly, that particular instances of the straight-rule inference schem a are only rationally acceptable if ‘F’ and ‘G’ are replaced by projectible predicates. Jackson launches a forthright attack on this consensus. He writes: ‘I will argue [that] there is no “new riddle of induction”, by arguing that all (consistent) predicates are projectible and that there is no paradox resulting from “grue” and like predicates’ (114). So in short, Jackson argues that the philosophical m ainstream has m ade a bad m istake in its appraisal of Goodm an’s ideas. He traces this m istake to three sources: ‘one, a tendency to conflate three different ways of defining “grue”; two, a lack of precision about just how, in detail, the “grue” paradox or new riddle of induction is supposed to arise; and three, a failure to note a counterfactual condition that governs the vast m ajority of our applications of the SR’. I turn to each of these three points in turn. 2. Jackson on the Definition of ‘Grue’ Jackson identifies three different ways of defining ‘grue’, each of which is found in the literature. The first two do not even pose a prima facie problem for the straight rule, he argues, and can be dealt with swiftly; 2 See for exam ple the papers in D. Stalker (ed.) Grue, Chicago: O pen Court, 1994. 486 Re-Reading the third does provide a prima facie problem , but it dissolves on closer exam ination. An exam ple of the first type of definition is: D1: x is grue iff x is green before T and blue thereafter where T is a chosen tim e in the future. As Jackson notes, on this definition ‘grue’ is an atem poral predicate, unlike green: an object cannot be grue at one tim e and fail to be grue at another tim e. Is ‘grue’ as defined by D1, or ‘grue1’ for short, an unprojectible predicate? Jackson argues, surely correctly, that the answer is ‘no’. To be grue1, an object m ust change from being green to blue at tim e T. If it turned out that all exam ined objects of a certain type, e.g., em eralds, did indeed change colour at tim e T, we would m ost likely conclude that there was som e reason for this, and thus that the em eralds we have not exam ined also underwent a colour change at tim e T. Em eralds would then sim ply be objects that undergo a colour change during their life- cycle, like tom atoes, the only difference being that they all change colour at one particular point in tim e. So although grue1 denotes a som ewhat odd property, it seem s perfectly projectible. An exam ple of the second type of definition is: D2: x is grue at t iff (x is green at t & t < T) or (x is blue at t & t ≥ T) As Jackson notes, grue as defined by D2—grue2 for short—is like green in being a tem poral predicate—an object m ay be grue2 at one tim e and not at another. Does the predicate grue2, when used with the straight rule of induction, lead to a paradox? According to Jackson, the answer is ‘no’. The widespread view to the contrary, he argues, arises from a confusion about whether ‘grue2’ is supposed to apply to enduring objects or to tim e-slices of enduring objects. Suppose we are doing straight-rule induction in relation to enduring objects. W e m ust then tem porally index the predicates that we are projecting. For enduring objects don’t have the property of being green Re-Reading 487 simpliciter, but rather being green at a particular tim e. O nly if this point is overlooked, Jackson argues, does the appearance of paradox arise when we use grue2 in conjunction with the SR. To see this, suppose the em eralds in our sam ple are all green at t1, where t1 is before T; by definition, they are also grue2 at t1.