Judgements of the relative plausibility of theories are based, I believe, on a sense of the structure of the implicit content of the theory. This content is the current state of the cognitive object underlying theories of that kind, the theory-family. There seem to be five main aspects of
a theory-family that influence asessments of plausibility and implausibility. I set them out as successive necessary conditions for making such a judgement.
1. The strongest condition necessary for a theory to be judged plausible is that it should represent a moment in the history of a theory-family at which there is a balance in the behavioural and material analogies. Precisely what is meant by a 'balance of analogies'7 The idea is this:
(i) Behavioural analogy: the better the imagined behaviour of the hypothetical generative or productive mechanism simulates the behaviour of the unknown real mechanism which actually produces the observed patterns, the more plausible is the theory which represents that moment in the development of the theory-family. The worse the simulation, the more implausible the theory.
(ii) Material analogy: the more fully the imagined properties of the hypothetical generative or productive mechanism match the essential properties of the source model, that is those properties that define the natural-kinds it represents, the more plausible is the theory. This condition on the material analogy ensures that the reality-determining natural-kind rules which express the ontological commitments of this theory-family are conserved.
Balance has to do with the way the behavioural and material analogies are restored when a theory-family is disturbed either by new empirical discoveries or by theoretical innovations, or both. This can be illustrated with the later history of the gas laws.
When Amagat discovered systematic divergencies in the behaviour of gases at high pressures from those predicted on the basis of Boyle's law, the simple point molecule conception of the mechanism responsible for the behaviour described by the law had to be revised. Amagat realized that gas molecules would be more plausible existents if they were thought of as having volume as well as position and momentum, since they were modelled on Newtonian particles which are essentially extended. At high pressures the volume of the molecules themselves ('a') would reduce the effective space in which they could move to 'V - a'. By so modifying his
90
Theory-Families
conception of a gas molecule, adjusting the positive and neutral components of the material analogy, Amagat changed the theory-family in such a way that its new state could be represented in a theory which took the form of a revised gas law 'p(V - a) = k'. This law was deductively consistent with his results. In short he restored the behavioural analogy between the way the real but unknown structure of the gas manifested itself in the new conditions he had created, and the imagined behaviour of the hypothetical nature of gas as expressed in the theory. A theoretical 'gas', imagined now to consist of spatially extended molecules, would behave more like real gases had been shown actually to behave in experiments. The analogies had been rebalanced, restoring plausibility to the modified theory as it represents a moment in the life of the theory- family.
The history of the discovery of the positron (described in detail in Hanson 1967) is a good example of the re-establishment of equilibrium aider a theory-led disturbance in the relevant theory-family. Dirac's theoretical work led him to postulate the possibility of positively charged particles of the same order of magnitude as electrons. The actual story of the eventual matching of Anderson's experimental results with Dirac's theoretical concept is rather complex, but from the perspective of this analysis it represents the restoration of the balance of behavioural and material analogies in that theory-family. This example is borderline in that both electrons and positrons are perhaps best considered to be Realm 3 entities, not of the same metaphysical status as ordinary kinds. It is worth a reminder that what I have in mind in this discussion is not the demonstration of the existence of a hitherto unknown kind of being, but the restoration of the key analogies, so that the behaviour of a hypothetical generative mechanism, which theory describes, is analogous to the behaviour of the real corresponding mechanism of nature, which experiment has revealed.
Since the body of data upon which intuitive judgements of plausibility and implausibility are based includes both observed patterns of phenomena and the content of the source analogue which specifies the natural-kinds of the proposed hypothetical entities, and since plausibility is adjusted to the strength of the analogies based upon both the observed patterns and the source analogue, the assessment of plausibility and implausibility is fully determined by the data. In this scheme the difficulty which Quine called the underdetermination of theory by data does not occur. It is easy to see now why, if data are confined to experimental or observational results, there seems to be underdetermination.
The first component in the concept of'plausibility of a theory' can be set out as follows:
A theory is plausible in so far as it represents a condition of a theory family in which the material and behavioural analogies are 'in balance'. A theory-family is in an imbalanced state, and the theory representing that moment is implausible, if either a behavioural advance has not been remedied by a change in material analogy (Amagat) or a change in material analogy has not been remedied by an advance in the behavioural analogy.
2. A theory-family in balance is the more plausible as its component analogies are strong.
Various intuitive formulae can be developed for the representation of the comparative strength of analogies. The strength of an analogy depends on the relations between its positive componem (likenesses between source and subject), its negative component (differences between source and subject) and its neutral component (those properties of the source whose likeness or difference from properties of the subject has yet to be explored). Generally an analogy is the stronger as the positive analogy outweighs the negative analogy, and both outweigh the neutral analogy. There are various matters of philosophical interest in defining 'measures' of weight and I postpone a detailed discussion of the assessment of the strength of analogies for a separate section. For the moment it will be enough to suggest that the more the comparison between source and subject, the two terms of the analogy, has been explored, and the more of the aspects studied that have
Scientific Explanation
91turned out to be likenesses, the stronger is the analogy.
Part of the difficulty that geologists found with the early versions of Wagener's hypothesis of continental dri~ and plate tectonics (Hallam 1973) seems to have been the fact that the analogy between continental masses and floating bodies had not been thoroughly explored. The neutral analogy was too large.
3. A theory-family which at a certain moment in its development is in balance, with strong analogies, is the more plausible in so far as the material analogy preserves the natural-kind rules for beings from whom the source analogue of the material analogy is derived. When Pasteur used analogical reasoning to develop his theory of disease he made an explicit comparison between suppuration and fermentation. Using the natural-kind conservation principle he inferred that if suppuration is like fermentation then there must be micro-organisms involved (bacteria) that are like yeasts. The causal agents of the infection of wounds are of the same natural-kind as the causal agents of the fermentation of liquors. And so the theory of disease as caused by micro- organisms is the more plausible. But when the attempt to balance the behavioural and material analogies calls for changes in the natural-kind rules characteristic of the source analogue in defining the entities of the theory the theory-family can slide into a state of implausibility. The putative reality of the imagined entities is undermined. It is this kind of difficulty that made it seem unreasonable to undertake a search for biological entelechies.
4. Since there are two independent sources of concepts for any theory-family, namely the analytical analogue (or model) and the source analogue (or model), the question of their interrelation can be raised. This provides us with a fourth feature that seems to go into judgements of relative plausibility, namely the degree of co-ordination between analytical analogue and source analogue. A theory-family in balance, with strong analogies, preserving natural-kind rules, is the more plausible in so far as its analytical and source analogues are co- ordinate, that is drawn from the same general conception of the empirical realm to which they
apply.
For instance in ethogenic psychology the analytic scheme for picking out relevant social patterns is based on an analogy between social action and staged performances, the dramaturgical model. A co-ordinate source analogue would be 'person as actor following a script'. An ethogenic account of a social event is the more plausible in so far as the explanatory theory, developed for the problematic behaviour in question, uses concepts based on those of actor and script, such as 'knowledge of role'. A measure of implausibility would infect an explanation for which the material analogy for the generative process was a socio-biological source, with concepts like 'gene selection', while the analytical analogue continued to enforce an analysis of behaviour in terms of concepts appropriate to the description of a staged performance.
This component of (im)plausibility can be given a Kantian turn. It is equivalent to the principle that the schematisms by which experience is ordered should be in one-to-one correspondence with the categories in terms of which one's whole cognitive apparatus is organized. But in the building of a plausible scientific theory it is by no means sure that such a correspondence can be achieved. It is a desideratum which has to be actively pursued.
The four components of the concept of '(im)plausibilty' could be mapped on to a four- dimensional space somewhat as in Figure 2.
92
Theory-Families
T2
g o o d n a t u r a l - k i n d p r e s e r v a t i o n
s t r o n g a n a l o g y
b a l a n c e d a n a l o g i e s
w e a k
~ ~
f a n a l o g yp o o r n a t u r a l - k i n d p r e s e r v a t i o n I
"" T'!
i m b a l a n c e d a n a l o g i e s Figure 2
A graphical representation of the concept of plausibility
The breadth of the trace represents the degree of co-ordination between analytical and source analogues. But there is a further consideration.
5. Popper and others have pointed out that our confidence in a theory grows with its successful passage of tests of various kinds. This idea was originally proposed for a deductivist conception of theories and their testing, but the same principle can be applied to the more elaborate structure described here. Meeting the above four desiderata can be thought of as passing tests. If a theory-family has the resources to develop so that it still meets the four desiderata late in its history so much the more plausible must that late moment of equilibrium appear as a particular theory. The fact that the kinetic theory of gases has survived successive modifications, but all within the basic overall scheme defined by its analytical and source models, is, I believe, a ground for holding that the most recent version merits our assent as a plausible theory. I offer these five conditions for plausibility as contributions to spelling the useful but the rather vague terms, 'disadvantage' and ' a d v a n t a g e ' . . . . But one must bear in mind that each successive 'theory' is a representation of a moment of equilibrium in a continuous history of adjustments within a theory-family. The Darwinian evolutionary theory- family and the theory-family developed around the concept of 'electron' can easily be shown to have evolved along the same lines as the kinetic theory-family.
If all five desiderata have been met, something more can be said about the nature of the
Scientific Explanation 93
the concepts in the theory-family at this or that moment in its history, shows that a theory-family is a structure. The changing balance between the behavioural and material analogies preserves this structure against the disruptive effect of certain disturbances. There are internal mechanisms of adjustment which maintain the integrity of the whole. The existence of these mechanisms suggests that a theory-family is not only a structure but could be usefully thought of as a system.
This suggests the possibility of further study of theory-families directed to a more detailed analysis of their system properties. It may also be the case that the social structure of scientific teams that work with theory-families mirrors the structure of those theory-families, and that the conversations within the team apropos their researches have system properties.
With the strict system of theory assessments displaced to the ethics of science, and the deductivist scheme of analysis reduced to a reflection of a mere heuristic and pragmatic aid to using parts of theory-families more effectively, plausibility, implausibility and complex structures of analogical relationships must fill the gap. But there are several ways in which (im)plausibility judgements differ from truth and falsity. For instance the grounds for plausibility judgements cannot be mapped on, to either of the grounds implicit in the traditional theories of truth. (Im)plausibility is not measured by degree of correspondence with the facts, nor is it an expression of the degree of coherence of the theory in question with other theories. Relative (im)plausibility assessments partake in some measure of each. There is a kind of correspondence in the use of observed patterns as a test for the behavioural analogy between the unknown real generators of those patterns and the hypothetical productive process imagined in accordance with the content of the theory-family. There is a kind of coherence in the requirement that our conceptions of the hypothetical mechanisms assumed in theories should be constrained by a material analogy to a source analogue which represents the kind of world we think we are exploring.
An important aspect of the concepts of truth and falsity is their use in defining the logical particles or truth-functions. If theory T~ is true and theory T2 is false then their logical conjunction T~ and T2 is false, and so on. If T~ is true then not-T~ is false. And there are many other familiar combinations. If 'true' and 'false' are transferred to the moral universe of the social order of scientists, what sort of meaning are we to give to the logical particles in theory- family discourse? I owe to Jonathan Bennett the idea of testing the way 'plausibility' and 'implausibility' work, and of more carefully establishing the meaning of 'and', 'or' and 'not' in so far as they are used to form complexes of theories.
Certain useful qualifications of the use of these concepts immediately become obvious, when Bennett's suggestion is pursued. Time must be taken into account. Two successive states of equilibrium of the same theory-family may be equally plausible, since their plausibility will be relative to the state of empirical knowledge and theoretical sophistication at any time. But to form a conjunction of the two theories, except in the purely historical sense that perhaps the scientific community once held the former and now holds the latter, makes no sense. Boyle's law and the Clausius model and Amagat's law and his model cannot be conjoined, since when the interface is 'zipped up' contradictions appear. A molecule cannot both have and lack volume.
This points up the fact that, in this way of looking at science, and the same goes for Lakatos' view as well, successive theories are determinates under the same determinable. The fact that Boyle's and Amagat's laws cannot be conjoined is the same kind of fact that blue and red cannot be jointly predicated of the same thing at the same time.
However, determinates can always be disjoined, and indeed one way of interpreting the determinable under which they fall is as the disjunction of its determinates. A disjunction of plausible theories within the same theory-family is that theory-family. And if the disjunction contains at least one plausible theory and it is the latest in the development of that family, then the family is plausible, by condition 5 above. Again a temporal condition must be imposed on the
94
Theory-Families
'molecular' structure.
If we try to form conjunctions or disjunctions of theories drawn from different theory- families, in any other sense than as mere catalogues of" what the French Academy, say, believed in 1775, we run into further complexities. A conjunction of plausible theories could be assessed for epistemic value, and the infection of an implausible conjunct represented only if there were some melding of source analogues (in Aronsonian terms, some degree of mapping on to a common ontology). Consilience (non-contradictory conjunction of empirical hypotheses on a combined data base) would increase plausibility of" the conjunction over any of its conjuncts taken separately only if ontological melding had occurred. There would be an increase in implausibility if there were an ontological clash. Further pursuit of these minutiae does not strike me as likely to prove fruitful, since the discussion so far has brought up two extra-logical matters which seem to exert a determining influence over how our intuitions should go. These are the temporal order of theories, as they reveal the way a theory-family has unfolded; a theory-family might unfold in various ways. The second matter is the degree to which a common ontology (common source analogue) can be created out of the theory-families relevant to each conjoined or disjoined theory. And this last seems to me to be a matter of content not form.
Finally, is the negation of a plausible theory implausible? It will depend a good deal on how much of the engendering theory-family is expressed in the theory, and thus to what parts of' the theory-family structure negation is to be applied. If it is no more than that which appears in the printed scientific paper, not much can be said. The negation of a plausible theory is nothing.
However, if the source model is contradicted (molecules are not point masses) then it might seem that something like the truth-functional relation between affirmed and negated theories does hold. At least the negation, in this last sense, of an implausible theory, may be