in cases where there is no relevant statutory rule , and the rule has to be drawn from cases , and not from a statute , the absence of an unalterable verbal formulation of the rule reduces the importance of the conventions of language , and makes it less natural to talk of interpretation , though sometimes the courts do behave just as they do with a statute , when , for some reason or another , a common law rule has achieved a settled formulation . but this is rather exceptional . the consequence is that problems of applicability which arise in the courts about common law rules can not be solved by interpretation - that is by a process of reasoning which attaches particular importance to linguistic considerations - for there is no text to interpret . solved they have to be , however , but by other types of reasoning . so it is that usually arguments as to whether an earlier case should be followed or distinguished do not rest primarily upon linguistic grounds ; they rest rather upon the use of analogy , and upon the discovery of factual similarity and difference between cases . but just as difficulties of interpretation , which seem to be difficulties about words , are really difficulties about the applicability of rules to facts , so also are many difficulties involved in the use of precedent . thus even if there is a measure of agreement about the ratio of an earlier case , an agreement , that is , as to what rule can be extracted from it , there may still be difficulty in the second task which confronts a court in using precedents - the task of deciding whether the rule does or does not fit the case before the court . neither being bound by statute , nor being bound by cases , absolves a court from this second task ; indeed it is only when a person or a court is to some degree or other bound by a rule that the second task becomes necessary at all . distinguishing cases , which consists in giving reasons why a rule in a case ought not to be followed or applied in a later case , is often conceived to be an indication that courts are not really bound ; in truth , earlier cases are distinguished , and have to be distinguished , just because they are binding , so that they ought to be followed unless a reason can be given for not following them ; in much the same way courts have to interpret statutes just because statutes are binding . the comparison between parliamentary and judicial legislation leads on to a second point . when we ask in what way parliament exercised its power to formulate a rule of the legal system , it is the existence of a text which enables an answer to be given without initial difficulty , except in rare and anomalous circumstances , and the lack of such a text which lies at the root of many of the difficulties when the same question is asked in relation to the judicial power of legislation . there is a natural temptation to seek for some technique for determining the ratio decidendi of a case which will repair the initial absence of a formal text : some formula such as read a Queen &apos;s printer &apos;s copy , which works well enough for parliament . there is a temptation to feel that there ought to be some formula , if only we could find it ; after all the whole doctrine of precedent depends upon the conception of the ratio decidendi , and it seems somehow absurd to accept the doctrine of precedent if we have to admit that we are not able to say what is the ratio decidendi of a particular case . the difficulty may perhaps be solved if it is realized that there are really two problems involved in the use of cases . the first is the problem of defining the ratio decidendi , that is to say defining what is meant by the ratio of a case . a satisfactory definition will indicate what a lawyer is to look for in his case . the second is the problem of determining the ratio decidendi . this is the problem of how to look , and not the problem of what to look for . it would indeed be odd if it was not possible to formulate a satisfactory definition of the expression ratio decidendi of a case ; indeed , failure here would indicate that it was high time to abandon the conception . it is quite another matter to suppose that there ought to be one technique or one set of rules , or one formula , which will serve as a general solution for the problem of determining what precisely is the ratio of a particular case . there may indeed be as many ways of finding the ratio of a case as there are ways of finding a lost cat ; certainly the ratio of some cases seems as elusive . defining the ratio decidendi of a case . in defining the ratio decidendi of a case , then , we must seek for a definition which will serve as an answer to the question . what am I to look for ? for purely legal purposes we may take it for granted that we should look in cases for a rule or rules of some kind or other . furthermore the term ratio decidendi is normally used to refer to some binding rule ( or rules ) which is to be found in decided cases - some rule which a later court ( appropriately placed in the hierarchy ) can not generally question . bearing all this in mind , a possible defining technique is to elucidate the judicial power to make binding rules , and to tell our questioner to seek for a rule ( or rules ) made within the ambit of this power - such a rule ( or rules ) will constitute the ratio of the case . this method of definition will have an obvious advantage , for it will be closely related to the purpose for which the conception of the ratio decidendi has been developed . for the conception only serves to point the distinction between the rule-making of judges which is intra vires a power to make binding rules , and the rule-making of judges which is ultra vires this power . furthermore the method suggested closely resembles the normal defining technique adopted to isolate the product of other law-making activities - for example , acts of parliament . and finally it leads to a very orthodox and unstartling result , for it is not in the least a novel technique . what then are the bounds upon the power of rule-making which is vested in judges ? the most important limitation is to be found in the principle which denies them the power to make binding rules except when those rules are relevant to the determination of actual litigation before the court in which they are empowered to sit . historically this limitation dates from the seventeenth century , when it became recognized that a court ought not to give official opinions upon hypothetical problems - a convention which has become refined and elaborated since then . as this convention came to be accepted an obvious corollary develops ; there must be some principle which has the effect of reducing the importance of enunciations of the law which have in fact been delivered by judges - either accidentally or deliberately - upon hypothetical issues . thus the conception of obiter dicta grows up ; obiter dicta are in some sense ultra vires enunciations of law . the distinction between such dicta and the elusive ratio decidendi is in essence a distinction between relevance and irrelevance , and much of the difficulty in elucidating the conception of the ratio decidendi arises from attempts to give a precise meaning to relevance in this context . without some criterion of relevance the judicial power of rule-making seems to have no limit , and in a country wedded to the conception of the rule of law there is naturally a desire to state with precision where the limit lies . limitations upon a rule-making power may be formal or substantial ; they may restrict the way in which rules are made , and they may restrict what rules are made . the power vested in the judges is subject to both kinds of limitation , but the concept of the ratio decidendi seems to embody only a formal limitation . this is that only a rule ( or rules ) acted upon in court can rank as a binding rule . once this primary condition is satisfied the rule will so rank , unless one of the various exceptions to the doctrine of precedent apply - for example the per incuriam rule . the rule becomes binding , subject to exceptions . the fact that the rule has been acted upon is the hallmark of relevance , and this may no doubt be expressed in a variety of different ways ; thus we talk of the rule applied , the reason for the decision , the grounds upon which the decision rested , the basis of the decision , and there is no particular advantage in adopting one of these formulations rather than another , for they are but variations upon a single theme . all state the primary formal limitation upon the judicial power , or , to put it another way , all state the manner and form in which the judicial power is exercised . they thus serve as definitions of the source of law under discussion - the rationes decidendi of cases - in much the same way as similar manner and form statements of the parliamentary power serve to define what a statute is . but , however we define the ratio decidendi of a case , we encounter difficulties in applying our definition which are much greater than those which accompany parliamentary law-making . the rule-making procedure of parliament operates on a text - a definite and settled verbal formulation of a rule or body of rules - and it is to the rules so drafted that legal validity is attached . with case law it is different ; we do not require the courts to draft the rules upon which they act . even where a judge does take some peculiar care to formulate a rule accurately and precisely , we do not usually treat such a formulation in the same way as a section in a statute , for the prerogative of judges is not to confer binding force upon a rule by formulating it and submitting the formulated rule to some procedure , but rather to decide cases by acting upon rules , without settling for the future the verbal form of the rule on the basis of a single application of it . the minimum required before a judge may be said to act upon a legal rule is that ( a ) he should have a rule in mind when he decides to act . this does not mean that he should have in mind a precise formulation of a rule ; a person may act upon a rule without thinking out a draft of the rule . ( b ) he should decide that the rule is applicable - that is to say he should decide that some fact or set of facts should be subsumed under the rule , and this will involve a task of classification . ( c ) he should deliberately so conduct himself that his conduct conforms to the conduct prescribed by the rule . in everyday life this acting upon a rule may be quite a casual process ; in the judicial process the convention is that the judge should show his working , and this produces a reflective acting upon a rule not so often met with outside the law and other special fields . and with this reflective acting upon rules which is characteristic of the judicial process goes the custom which the courts have adopted of justifying the action taken by an opinion delivered openly in court , which opinion provides the best possible evidence of the rule upon which the court did act . it will be noted that to say that a person acted upon a rule is not to assert anything about the psychological motivation of his action . recognition of this has wide implications in legal theory . furthermore , in general , a person may act upon a rule notwithstanding the fact that he may himself be the originator of the rule , as will sometimes be the case in judicial decisions . 