It is taken for granted that all respectable reasoning can be formalised, and that any sense of reasonableness which cannot be articulated into definite and statable reasons in standard form is untrustworthy and suspect. Thus Hare, arguing against Toulmin's ``evaluative'' inference, says1 ``If this is to be a valid inference, there must be a rule of inference (say R) to the effect that inferences of this form are valid''. It is easy to see the attraction of such a doctrine. Leaving matters to people's subjective sense of what is reasonable seems to open the floodgates to unreason and prejudice. Only by having a definite written rule can we combat the arguments of stupid and opinionated men. The programmes of Carnap and Hempel, if they could be carried out, would secure us against much that is false and phoney, which less rigid criteria are quite unable to keep out.
Unfortunately much besides the phoney is excluded. The humane disciplines have not been amenable to being tidied up. Historical explanations as actually put forward by practising historians do not fit the schema of historical explanation put forward by Hempel, and claims that, appearances notwithstanding, actual instances do really conform to the pattern laid down, are either made good analytically and trivially, or are full of Procrustean menace. It is possible, for each historical explanation or inference, to posit a covering law or rule of inference tailored to fit the case in point, so that the explanation or inference in question is trivially an instance of a covering law or rule of inference. One merely repeats the  particular explanation or inference in a pseudo-general form. I have my own doubts whether even this little is legitimate,2 but in any case it must be obvious that not much is to be achieved by such means. If a philosopher is unwilling to legitimate every explanation and every inference by positing the corresponding law or rule of inference, and in so far as he is unable, as philosophers have hitherto been unable, to produce a plausible account of how actual arguments could be shown to fit into antecedently determined schemata, he must be prepared to extrude such arguments from the corpus of respectable knowledge. This many philosophers are prepared to do. History is not a respectable discipline in their eyes. Nor is Law, nor is Literature. These are soft subjects, where anything goes, without objective standards of argument; and unless and until these subjects can be rationally reconstructed into proper disciplines with formally defined concepts and properly formulated canons of inference, they are not worthy to be spoken of in the same breath as respectable subjects like the physical sciences, and mathematics itself.
The trouble is that not even the respectable disciplines are respectable enough. Many attempts have been made to formalise scientific inference, but so far without success. Scientific theories, of course, have been formalised, in a way in which historical theories have not; but it has not been possible to reduce all scientific inferences to instances of a set of definite and statable rules. There are canons of induction, but they are not complete. Always it is necessary to supplement them with common sense. We have many useful procedures for the elimination of falsehood, and Popper has shown what power the falsifiability principle has. But though we can knock down false theories according to rules, we have as yet no complete set of rules for thinking up true ones. That demands nous.
The failure of an attempt does not prove its impossibility, and a philosopher might still hope that a completely formal theory of inductive inference might one day be achieved. We might doubt, but could not conclusively confute, this claim; and though, so long as scientific inference is not completely formalised, it would be churlish to condemn the humanities for their informality, we might still be uneasy and feel that they had got only a temporary reprieve. The case, however, is very much altered by the fact that even in mathematics, the most formal of all disciplines, the formalist programme cannot be carried through to completion. This follows from G”del's theorem. Given any respectable formalisation of mathematics, with definite axioms and definite rules of inference, we can produce an informal argument which will convince any mathematician of the truth of a proposition which cannot be proved from those axioms by means of those rules of inference. This to my mind is decisive. If not even deductive inference can be completely formalised, then it cannot be a reproach to any other sort of inference that it cannot be formalised either. The programme of the formalisers loses its spell, and it cannot be an objection to our using the word `reasonable' that we are unable to give complete criteria of when it may be properly applied, 
This I cannot do. What I can do, in a partial and inadequate attempt to meet the challenge, is to offer an account of how reasonableness is embodied in many men, an account of the social structure of reasonableness. It is, so to speak, an exercise in logical snobbery. Most people make some discriminations between more and less reasonable men, and the most reasonable men make the most coherent and most fine discriminations. More fully, I claim first that a great many people are sometimes able to say that one argument is reasonable and another is not, or that one argument is more reasonable than another, or that one person is being reasonable and another unreasonable, or one is being more reasonable than another. I claim, secondly, that the judgements of different people on the same arguments, or on the same persons arguing, tend to coincide, though far from completely coinciding. With closely balanced arguments and arguers support is apt to divide more or less equally, but there are many extreme cases, which we could regard as paradigm cases, where almost every one is agreed. This is enough to show that judgements of reasonableness are not matters of personal whim, varying from man to man arbitrarily. My third claim is that, with persons, being reasonable turns out in fact to be a dispositional rather than an episodic quality. Although we can all think of counter-examples, where a man whose judgement we all respect gets hold of the wrong end of the stick and cannot be brought to see sense, or where some highly hysterical woman for once shows herself surprisingly sane, on the whole our experience is that people who have been reasonable in the past will turn out to be reasonable in the future also, so much so that we are prepared to suspend our own immediate unfavourable assessments of their reasonableness on a particular issue in the anticipation that further consideration or fuller understanding will counter first impressions. That is, although we start with the individual's immediate assessment of reasonableness, this assessment is capable of correction, thanks to the transfer of the quality from particular arguments to particular arguers.
In the same way as we allow our immediate assessments of the reasonableness of people to be confronted with and sometimes corrected by our earlier and more considered assessments, so we allow our own assessments to be modified by those of others whom we respect. This is possible on account of the fourth fact on which I build, namely that wise men think alike, esthloi men gar haplos, pantodapoi kakoi3 Again I stress that I do not claim unanimity on behalf of the wise, nor do I deny that the learned world is riven by controversy : but I do claim that reasonable men do tend to agree more than they disagree, particularly in their assessments of the  reasonableness of third parties. The appearance to the contrary is, I think, largely illusory. It is like the law. We have the impression that the law is very uncertain, because we hear of so many doubtful cases; but we forget the innumerably many cases that are never brought to court, because they are so clear that the parties settle without recourse to adjudication. So too in general, we have the impression of discord because controversy provokes men to give utterance, and attracts attention, while agreement is silent and little noticed. And that there is a large measure of underlying agreement can be argued from the fact that we have more or less successfully embodied our assessments of reasonableness in social forms. It is open to the critic to object that what people are agreeing about is not of any importance, or is too much, or indeed altogether, determined by social conditioning; but that the judgements of the ``wise'' do tend to coincide can hardly be denied.
With these four facts we can establish and refine a category of reasonable men who more or less, though always imperfectly, embody the ideal of the Reasonable Man. The ideal gains substance and shape from its embodiments, rather than vice versa. And I would maintain further that, anchored in the way I have described, the concept of the Reasonable Man has a fundamental part to play in our conceptual structure. Although in the nature of the case one cannot produce a proof that the Reasonable Man is always right and worthy to be believed, I can, I think, show that he has many of the marks of being right.
First, there is the coherence and stability of the reasonable men, the wise. They tend to agree in their assessments, and they tend to assess one another favourably. The latter, perhaps, is not a very positive recommendation. Back-scratching is a fairly common characteristic, and not a particularly endearing one to outsiders. Nevertheless, the contrary trait, scratching out one another's eyes, is even less endearing, and a positive disqualification. The fact that reasonable men are not thus disqualified from being collectively right, should be noted if not dwelt on. The former, the way in which reasonable men agree in their assessments of third parties, is a strong argument in favour of their being on to something real, and not just each exercising his fancy. Thus even in the case of perception, where there is a strong tradition of the democracy of all observers, we are willing on occasion to give credence to the reports of a minority provided they can agree among themselves. There are some organic compounds which to certain people have a bitter taste, whereas to everybody else they are tasteless. We do not accuse either part of the population of being mistaken, although their discriminations and assimilations are not shared by all. If all seventh sons of seventh sons were able to agree in their descriptions of the ghosts they saw, we should have to concede that they did possess second sight, and on similar terms we should allow sixth sense to subjects of experiments in Extra-Sensory Perception.
The discriminations of the Reasonable Man are in better case. The resemblance is with flicker photometry.9 ``Only observers, or groups of  observers, who find for these solutions a ratio of transmission factors, the `Y/B ratio', which is close to unity (at any rate to within 5 per cent) should make observations in heterochromatic photometry'',9 and these form only a minority of the population.6 Nevertheless, not only do these observers obtain results consistent as between one occasion and another, as well as between one observer and another,9 but their discriminations tie in with those of observers outside the &eactue:lite. It is a rational procedure to throw away the readings of some, because they are colour blind, and of others, because their readings show a marked deviation from the mean in favour of blue or of yellow light. And though the majority will not reach the same results as the selected minority in difficult cases, there are more clear-cut cases where the minority view will be accepted by nearly everyone. Similarly with reasonable men. They are not merely a self-appointed &eactue:lite. Although they are not simply elected by the suffrages of the many, and their findings are often not capable of independent validation by any chance met featherless biped, yet their judgments are sufficiently consonant with the opinions of the many to clear them of any suspicion of being merely the eccentricities of eggheads. Though reasonable men have often rejected popular views as unreasonable, there is enough contact and overlap of judgements to justify the claim that the refined assessments of the wise may be more sophisticated than those of simple men, but are basically the same in nature and intention.
The corrigibility of assessments of reasonableness preserves them from some of the more purely philosophical criticisms which would prove fatal. No quasi-naturalistic fallacy is being committed, because we are offering an explication not of the meaning of the word `reasonable' but of how its applications work out, and no Cephalus fallacy is being involved because we are not offering complete, criteria for its applicability. That is, it always makes sense to ask whether a particular argument or particular person, generally accepted, or accepted by those best qualified to judge, as reasonable is in fact reasonable, and it is always conceivable, until at least the arguments have been gone into, that the answer may be `No'. Again, many of the traditional arguments against intuitionism do not seem to apply here. In saying that most people sometimes have a feeling of a particular argument's being reasonable, or that some people often have a well developed sense of what is reasonable and what is not, no perceptual analogy is being made. The content of the feeling that something is reasonable is given by the `that . . .' clause, and it is to miss the point to ask how we are to go from the feeling to the judgment itself, or to point out that it is logically possible to have the feeling although the argument in question was not in fact reasonable : for the feeling is the tentative articulation of  the judgement, and if ever we came to recognise that something we had previously thought to be reasonable is not reasonable after all, we cease to feel that it is.
To the demand, then, ``Shew us the Reasonable Man'', I reply, Establishment man that I am, by pointing to the structure of our society, and the fact that although there is no completely reasonable man, there are some fairly reasonable men, who are generally recognised as such by the consensus of opinion and who, to a much higher degree, recognise one another as such. A fairly widely distributed, though not very subtle, ability to discriminate on a few occasions between reasonable and unreasonable arguments is refined and corrected to produce the highly discriminating, though for the most part informal, assessments of reasonableness which we find in our society. The fact that we can engage in, and sometimes settle, disputes for which no formal decision-procedures exist and that social activities which depend on the possibility of this can be carried on, is the final vindication of the Reasonable Man : and conversely, if nearly all men are reasonable some of the time, some more than others, some quite a lot of the time but none all of the time, then we should expect society to take the shape which in fact it does.
The fallibility of the Reasonable Man accounts for much else. It is this that is responsible for the dialectical character of much of our thinking. If I were infallible, my soliloquies would be above reproach: but being liable to err, as we all are, it is wise to talk things over with somebody else as a check, and being fairly often mistaken, as I am, it is unlikely that I shall be able to talk for long without my listener wanting to interrupt.  Argument is not, as many philosophers have been inclined uncritically to assume, a matter of a solitary ego cogitating about an impersonal it, and developing a theme in a series of articulate and logically connected propositions forming a continuous monologue: it is rather a two-person activity, yours as much as mine, each of us probing the weak spots in the other's case, each modifying and correcting his position or his formulation of it in the light of the other's objections or misinterpretations. With the partial exception of mathematics, where some of the arguments are almost beyond question, most of our reasoning is dialectical in structure. It proceeds through claim and counter-claim, objection and rebuttal, considering first one side of the case and then the other; and the typical connective of argument is not `therefore' but `but'.
The dialectical nature of our thinking imposes further conditions. It not only reflects our fallibility, so that reasonable men can find themselves in disagreement---if they did not disagree there would be nothing for them to argue about---but requires that there should also be other beliefs shared by both parties---if they did not agree at all, there would be nothing for them to argue from.8 Not only some beliefs but certain goals must be held in common.9 For a dialogue is a co-operative venture as well as a dispute, a talking with as well as a talking against, a conversation as well as a debate. From the clash of opposing opinions in the course of a discussion we seek to shake loose the falsehoods among them and winnow out the truth, but the interchange becomes a sterile and empty altercation unless both parties are united by the common purpose of seeking truth, so that each would rather modify his position for the sake of truth than maintain it by means of turning a deaf ear to all argument for the sake of his amour propre. This is the distinction between dialektike and eristike, between rational discussion and the mere scoring of debating points, that Plato draws in the seventh book of the Republic.
10 The differences between dialektike and eristike, because they are impossible to characterize except in vaguely moral terms, have been overlooked by philosophers who have seldom realised that argument is properly between friends, not enemies, and that though one may well begin an argument with the intention of vindicating the rightness of one's views, it is also a condition of having a proper argument, that one should bring to it a willingness to be convinced of the wrongness of one's views, should convincing reasons be adduced. Only if each party to an argument recognises himself, and believes the other, to be subject to the common tribunal of reason, in the sense that each would rather reach the right solution than merely have his own  way, is it worth while arguing at all. There is an essential non-egocentricity which being reasonable requires, and which also is a sine qua non of a man's engaging in any common pursuit. A rational agent cannot be completely solipsistic in his thinking. The entirely self-centred subject suffers from a logical original sin that precludes him from reaching the level of rational discourse. By a certain grace, which we cannot explain, human beings are enabled to burst out of their egocentric predicament and are not completely cut off from their fellows, but can commune with them, and together with them can on occasion participate in the elucidation of what is reasonable and right.
The rules for the reasonable conduct of a dialogue cannot be fully formulated, and therefore no complete classification of sound arguments is possible. Nevertheless, a useful, although partial, classification of types of argument can, be built up on the basis of the extent of agreement in purpose presupposed by the argument and the sanctions for failure to play fair. The minimal agreement for discourse to be possible is that both parties should desire to communicate, and there are some rules failure to observe which involves a breakdown of communication. The corresponding type of argument is deductive argument. It presupposes only this minimal measure of agreement, and if a person refuses to acknowledge the force of deductive inferences, then communication with him becomes impossible, and we can only say, with Plato, let us separate, ean chairein. It is natural to extend agreement to cover not only the bare condition of consistency, without which on any individual occasion communication would break down, but the stronger condition of a common external world, without which common context we could not come to communicate with one another. If there is any common external world, the appeal to the evidence of the senses, ``Go, look and see'', must be an appropriate method of resolving some disputes; and if that common external world is to serve as a backcloth to effective communication, it must have a soi-disant consistency of its own that would justify the use of inductive inferences in argument. Nor need we stop short at things. We have an idea of what it is to be a person, since if we were not persons we could not be, as we are, communicating with each other, and on the assumption of other persons being of like passions with oneself, one can hazard, though arguments about other people are more than usually frail, a conjecture on how they feel and what their reactions to given circumstances are going to be. Further fusion of purpose and deeper levels of rapport are possible, right on to the extreme, ideal case, where the parties forget their separate interests altogether, and want nothing so much as to merge their individual viewpoints and achieve a true marriage of minds in a common apprehension of the truth.
Much academic writing has suffered because the author has not kept clearly in view what sort of discourse he is composing and what sort of reader he is addressing, but keeps breaking off in the course of his argument to give qualifications or justifications appropriate to quite a different type  of argument addressed to a different audience. In philosophy the confusion has been worse than merely stylistic. Instead of being content to deal with one sort of problem proposed by one sort of enquirer, the philosopher has felt impelled to try to answer in one breath every question that any one could raise, and meet every conceivable objection. The philosopher must take on all comers, and therefore is unable to avail himself of the agreement which underlies any particular argument, but must confine himself to reasoning that is coercive on everybody, that is to deductive reasoning, and with this alone must set himself against the world. Thus it is that, since the time of Descartes, philosophy has been almost exclusively concerned with the problems of a solitary and suspicious don, wrapt in his own thoughts, prepared to be as unreasonably sceptical as his forebears were foolishly credulous, who occasionally emerges from his bedroom to make sure that his study furniture is still where it was the night before, and is tempted at intervals throughout the day to peer back round the bedroom door just to make absolutely sure that it has really not become inhabited by a horde of pink elephants. These problems have their importance. Epistemological persecution mania is a distressing complaint, and one by which we are all afflicted sometimes. But not at all times. Loneliness is not the only condition of mankind, and our sociability is as philosophically significant as our solitude. To be a philosopher one does not have to be an anchorite, solely concerned with one's own experience, and anxious only to use language as a means of mapping it for one's private delectation: one can also be a member of a community of colleagues, all of whom mean well some of the time, but all liable to mistakes and errors. Not all their arguments are sound, nor all their objections well-founded : but it is a manageable task to deal with the actual objections people actually advance, whereas it is a hopeless endeavour to answer all the objections one can imagine an advocatus diaboli bringing up: and criticisms can be constructive as well as destructive. And therefore in our philosophy we should base ourselves on a different view of human nature, fallible still, but not necessarily false, reasonable, though not necessarily right: we should continue to realise that our colleagues and companions may be wrong, but should no longer half consciously assume that they must be: we should be wary of human judgement, but not despair of it, and should seek to correct rather than confute. For, in the last resort, human judgement is all that we have to go by: and we can only trust that it is possible for us to be reasonable, and sometimes even to be right.
J. R. LUCAS
Merton College, Oxford.
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1. Philosophical Quarterly, 1950-51, p. 374.
2. See Lucas, ``The Lesbian Rule'', Philosophy 1955.
3. Nicomachean Ethics Bk. II, 1106 b 35.
4. See J.Guild, Proceedings of the Optical Convention 1926, Vol. 1, pp. 88-104.
5. J.W.T.Walsh, Photometry, p. 311, based on Transactions of the Illuminating Engineering Society of New York, 23, 1928, p. 361.
6. Guild, op. cit., p. 99; out of 130 observers, only 25 were ``normal''.
7. Contrast Walsh, op. cit., p. 293, on the direct comparison of colour differences without the use of a flicker photometer: `` . . . When, however, the colour difference is increased beyond this limit, not only do different observers disagree markedly, but the same observer becomes inconsistent from day to day''.
8. Cf. Lucas, ``On Not Worshipping Facts'', Philosophical Quarterly, 1958, p. 146.
9. Cf. David Pole, The Later Philosophy of Wittgenstein, London, 1958, p. 59. At the time of writing this paper, I did not have the opportunity of considering Mr. Pole's subsequent book, Conditions of Rational Enquiry, which deals with the theme of this paper in a slightly different, but much more adequate, fashion.
10. Republic 539 b.e ; for the word, see Sophist 23le.
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