Principia mathematica, vol. 1 (of 3)

THE theory of logical types, to be explained in the present Chapter, recommended itself to us in the first instance by its ability to solve certain contradictions, of which the one best known to mathematicians is Burali-Forti's concerning the greatest ordinal. But the theory in question is not wholly dependent upon this indirect recommendation: it has also a certain consonance with common sense which makes it inherently credible. In what follows, we shall therefore first set forth the theory on its own account, and then apply it to the solution of the contradictions.

An analysis of the paradoxes to be avoided shows that they all result from a certain kind of vicious circle[12]. The vicious circles in question arise from supposing that a collection of objects may contain members which can only be defined by means of the collection as a whole. Thus, for example, the collection of propositions will be supposed to contain a proposition stating that "all propositions are either true or false." It would seem, however, that such a statement could not be legitimate unless "all propositions" referred to some already definite collection, which it cannot do if new propositions are created by statements about "all propositions." We shall, therefore, have to say that statements about "all propositions" are meaningless. More generally, given any set of objects such that, if we suppose the set to have a total, it will contain members which presuppose this total, then such a set cannot have a total. By saying that a set has "no total," we mean, primarily, that no significant statement can be made about "all its members." Propositions, as the above illustration shows, must be a set having no total. The same is true, as we shall shortly see, of propositional functions, even when these are restricted to such as can significantly have as argument a given object . In such cases, it is necessary to break up our set into smaller sets, each of which is capable of a total. This is what the theory of types aims at effecting.

The principle which enables us to avoid illegitimate totalities may be stated as follows: "Whatever involves all of a collection must not be one of the collection"; or, conversely: "If, provided a certain collection had a total, it would have members only definable in terms of that total, then the said collection has no total." We shall call this the "vicious-circle principle," because it enables us to avoid the vicious circles involved in the assumption of illegitimate totalities. Arguments which are condemned by the vicious-circle principle will be called "vicious-circle fallacies." Such arguments, in certain circumstances, may lead to contradictions, but it often happens that the conclusions to which they lead are in fact true, though the arguments are fallacious. Take, for example, the law of excluded middle, in the form "all propositions are true or false." If from this law we argue that, because the law of excluded middle is a proposition, therefore the law of excluded middle is true or false, we incur a vicious-circle fallacy. "All propositions" must be in some way limited before it becomes a legitimate totality, and any limitation which makes it legitimate must make any statement about the totality fall outside the totality. Similarly, the imaginary sceptic, who asserts that he knows nothing, and is refuted by being asked if he knows that he knows nothing, has asserted nonsense, and has been fallaciously refuted by an argument which involves a vicious-circle fallacy.

In order that the sceptic's assertion may become significant, it is necessary to place some limitation upon the things of which he is asserting his ignorance, because the things of which it is possible to be ignorant form an illegitimate totality. But as soon as a suitable limitation has been placed by him upon the collection of propositions of which he is asserting his ignorance, the proposition that he is ignorant of every member of this collection must not itself be one of the collection. Hence any significant scepticism is not open to the above form of refutation.

The paradoxes of symbolic logic concern various sorts of objects: propositions, classes, cardinal and ordinal numbers, etc. All these sorts of objects, as we shall show, represent illegitimate totalities, and are therefore capable of giving rise to vicious-circle fallacies. But by means of the theory (to be explained in Chapter III) which reduces statements that are verbally concerned with classes and relations to statements that are concerned with propositional functions, the paradoxes are reduced to such as are concerned with propositions and propositional functions. The paradoxes that concern propositions are only indirectly relevant to mathematics, while those that more nearly concern the mathematician are all concerned with propositional functions. We shall therefore proceed at once to the consideration of propositional functions.

By a "propositional function" we mean something which contains a variable , and expresses a proposition as soon as a value is assigned to . That is to say, it differs from a proposition solely by the fact that it is ambiguous: it contains a variable of which the value is unassigned. It agrees with the ordinary functions of mathematics in the fact of containing an unassigned variable: where it differs is in the fact that the values of the function are propositions. Thus e.g. " is a man" or "" is a propositional function. We shall find that it is possible to incur a vicious-circle fallacy at the very outset, by admitting as possible arguments to a propositional function terms which presuppose the function. This form of the fallacy is very instructive, and its avoidance leads, as we shall see, to the hierarchy of types.

The question as to the nature of a function[13] is by no means an easy one. It would seem, however, that the essential characteristic of a function is ambiguity. Take, for example, the law of identity in the form " is ," which is the form in which it is usually enunciated. It is plain that, regarded psychologically, we have here a single judgment. But what are we to say of the object of the judgment? We are not judging that Socrates is Socrates, nor that Plato is Plato, nor any other of the definite judgments that are instances of the law of identity. Yet each of these judgments is, in a sense, within the scope of our judgment. We are in fact judging an ambiguous instance of the propositional function " is ." We appear to have a single thought which does not have a definite object, but has as its object an undetermined one of the values of the function " is ." It is this kind of ambiguity that constitutes the essence of a function. When we speak of "" where is not specified, we mean one value of the function, but not a definite one. We may express this by saying that "" ambiguously denotes , , , etc., where , , , etc., are the various values of "."

When we say that "" ambiguously denotes , , , etc., we mean that "" means one of the objects , , , etc., though not a definite one, but an undetermined one. It follows that "" only has a well-defined meaning (well-defined, that is to say, except in so far as it is of its essence to be ambiguous) if the objects , , , etc., are well-defined. That is to say, a function is not a well-defined function unless all its values are already well-defined. It follows from this that no function can have among its values anything which presupposes the function, for if it had, we could not regard the objects ambiguously denoted by the function as definite until the function was definite, while conversely, as we have just seen, the function cannot be definite until its values are definite. This is a particular case, but perhaps the most fundamental case, of the vicious-circle principle. A function is what ambiguously denotes some one of a certain totality, namely the values of the function; hence this totality cannot contain any members which involve the function, since, if it did, it would contain members involving the totality, which, by the vicious-circle principle, no totality can do.

It will be seen that, according to the above account, the values of a function are presupposed by the function, not vice versa. It is sufficiently obvious, in any particular case, that a value of a function does not presuppose the function. Thus for example the proposition "Socrates is human" can be perfectly apprehended without regarding it as a value of the function " is human." It is true that, conversely, a function can be apprehended without its being necessary to apprehend its values severally and individually. If this were not the case, no function could be apprehended at all, since the number of values (true and false) of a function is necessarily infinite and there are necessarily possible arguments with which we are unacquainted. What is necessary is not that the values should be given individually and extensionally, but that the totality of the values should be given intensionally, so that, concerning any assigned object, it is at least theoretically determinate whether or not the said object is a value of the function.

It is necessary practically to distinguish the function itself from an undetermined value of the function. We may regard the function itself as that which ambiguously denotes, while an undetermined value of the function is that which is ambiguously denoted. If the undetermined value is written "," we will write the function itself "." (Any other letter may be used in place of .) Thus we should say " is a proposition," but " is a propositional function." When we say " is a proposition," we mean to state something which is true for every possible value of , though we do not decide what value is to have. We are making an ambiguous statement about any value of the function. But when we say " is a function," we are not making an ambiguous statement. It would be more correct to say that we are making a statement about an ambiguity, taking the view that a function is an ambiguity. The function itself, , is the single thing which ambiguously denotes its many values; while , where is not specified, is one of the denoted objects, with the ambiguity belonging to the manner of denoting.

We have seen that, in accordance with the vicious-circle principle, the values of a function cannot contain terms only definable in terms of the function. Now given a function , the values for the function[14] are all propositions of the form . It follows that there must be no propositions, of the form , in which has a value which involves . (If this were the case, the values of the function would not all be determinate until the function was determinate, whereas we found that the function is not determinate unless its values are previously determinate.) Hence there must be no such thing as the value for with the argument , or with any argument which involves . That is to say, the symbol "" must not express a proposition, as "" does if is a value for . In fact "" must be a symbol which does not express anything: we may therefore say that it is not significant. Thus given any function , there are arguments with which the function has no value, as well as arguments with which it has a value. We will call the arguments with which has a value "possible values of ." We will say that is "significant with the argument " when has a value with the argument .

When it is said that e.g. "" is meaningless, and therefore neither true nor false, it is necessary to avoid a misunderstanding. If "" were interpreted as meaning "the value for with the argument is true," that would be not meaningless, but false. It is false for the same reason for which "the King of France is bald" is false, namely because there is no such thing as "the value for with the argument ." But when, with some argument , we assert , we are not meaning to assert "the value for with the argument is true"; we are meaning to assert the actual proposition which is the value for with the argument . Thus for example if is " is a man," (Socrates) will be "Socrates is a man," not "the value for the function ' is a man,' with the argument Socrates, is true." Thus in accordance with our principle that "" is meaningless, we cannot legitimately deny "the function ' is a man' is a man," because this is nonsense, but we can legitimately deny "the value for the function ' is a man' with the argument ' is a man' is true," not on the ground that the value in question is false, but on the ground that there is no such value for the function.

We will denote by the symbol "(" the proposition " always[15]," i.e. the proposition which asserts all the values for . This proposition involves the function , not merely an ambiguous value of the function. The assertion of , where is unspecified, is a different assertion from the one which asserts all values for , for the former is an ambiguous assertion, whereas the latter is in no sense ambiguous. It will be observed that "(" does not assert " with all values of ," because, as we have seen, there must be values of with which "" is meaningless. What is asserted by "(" is all propositions which are values for ; hence it is only with such values of as make "" significant, i.e. with all possible arguments, that is asserted when we assert "(." Thus a convenient way to read "(" is " is true with all possible values of ." This is, however, a less accurate reading than " always," because the notion of truth is not part of the content of what is judged. When we judge "all men are mortal," we judge truly, but the notion of truth is not necessarily in our minds, any more than it need be when we judge "Socrates is mortal."

Since "(" involves the function , it must, according to our principle, be impossible as an argument to . That is to say, the symbol "" must be meaningless. This principle would seem, at first sight, to have certain exceptions. Take, for example, the function " is false," and consider the proposition "( is false." This should be a proposition asserting all propositions of the form " is false." Such a proposition, we should be inclined to say, must be false, because " is false" is not always true. Hence we should be led to the proposition i.e. we should be led to a proposition in which "( is false" is the argument to the function " is false," which we had declared to be impossible. Now it will be seen that "( is false," in the above, purports to be a proposition about all propositions, and that, by the general form of the vicious-circle principle, there must be no propositions about all propositions. Nevertheless, it seems plain that, given any function, there is a proposition (true or false) asserting all its values. Hence we are led to the conclusion that " is false" and " is false" must not always be the values, with the arguments and , for a single function " is false." This, however, is only possible if the word "false" really has many different meanings, appropriate to propositions of different kinds.

That the words "true" and "false" have many different meanings, according to the kind of proposition to which they are applied, is not difficult to see. Let us take any function , and let be one of its values. Let us call the sort of truth which is applicable to "first truth." (This is not to assume that this would be first truth in another context: it is merely to indicate that it is the first sort of truth in our context.) Consider now the proposition (. If this has truth of the sort appropriate to it, that will mean that every value has "first truth." Thus if we call the sort of truth that is appropriate to ( "second truth," we may define " has second truth" as meaning "every value for has first truth," i.e. "(.( has first truth)." Similarly, if we denote by "(" the proposition " sometimes," i.e. as we may less accurately express it, " with some value of ," we find that ( has second truth if there is an with which has first truth; thus we may define " has second truth" as meaning "some value for has first truth," i.e. "(." Similar remarks apply to falsehood. Thus " has second falsehood" will mean "some value for has first falsehood," i.e. "(," while " has second falsehood" will mean "all values for have first falsehood," i.e. "(." Thus the sort of falsehood that can belong to a general proposition is different from the sort that can belong to a particular proposition.

Applying these considerations to the proposition "( is false," we see that the kind of falsehood in question must be specified. If, for example, first falsehood is meant, the function " has first falsehood" is only significant when is the sort of proposition which has first falsehood or first truth. Hence "( is false" will be replaced by a statement which is equivalent to "all propositions having either first truth or first falsehood have first falsehood." This proposition has second falsehood, and is not a possible argument to the function " has first falsehood." Thus the apparent exception to the principle that "" must be meaningless disappears.

Similar considerations will enable us to deal with "not-" and with " or ." It might seem as if these were functions in which any proposition might appear as argument. But this is due to a systematic ambiguity in the meanings of "not" and "or," by which they adapt themselves to propositions of any order. To explain fully how this occurs, it will be well to begin with a definition of the simplest kind of truth and falsehood.

The universe consists of objects having various qualities and standing in various relations. Some of the objects which occur in the universe are complex. When an object is complex, it consists of interrelated parts. Let us consider a complex object composed of two parts and standing to each other in the relation . The complex object "-in-the-relation--to-" may be capable of being perceived; when perceived, it is perceived as one object. Attention may show that it is complex; we then judge that and stand in the relation . Such a judgment, being derived from perception by mere attention, may be called a "judgment of perception." This judgment of perception, considered as an actual occurrence, is a relation of four terms, namely and and and the percipient. The perception, on the contrary, is a relation of two terms, namely "-in-the-relation--to-," and the percipient. Since an object of perception cannot be nothing, we cannot perceive "-in-the-relation--to-" unless a is in the relation to . Hence a judgment of perception, according to the above definition, must be true. This does not mean that, in a judgment which appears to us to be one of perception, we are sure of not being in error, since we may err in thinking that our judgment has really been derived merely by analysis of what was perceived. But if our judgment has been so derived, it must be true. In fact, we may define truth, where such judgments are concerned, as consisting in the fact that there is a complex corresponding to the discursive thought which is the judgment.

That is, when we judge " has the relation to ," our judgment is said to be true when there is a complex "-in-the-relation--to-," and is said to be false when this is not the case. This is a definition of truth and falsehood in relation to judgments of this kind.

It will be seen that, according to the above account, a judgment does not have a single object, namely the proposition, but has several interrelated objects. That is to say, the relation which constitutes judgment is not a relation of two terms, namely the judging mind and the proposition, but is a relation of several terms, namely the mind and what are called the constituents of the proposition. That is, when we judge (say) "this is red," what occurs is a relation of three terms, the mind, and "this," and red. On the other hand, when we perceive "the redness of this," there is a relation of two terms, namely the mind and the complex object "the redness of this." When a judgment occurs, there is a certain complex entity, composed of the mind and the various objects of the judgment. When the judgment is true, in the case of the kind of judgments we have been considering, there is a corresponding complex of the objects of the judgment alone. Falsehood, in regard to our present class of judgments, consists in the absence of a corresponding complex composed of the objects alone. It follows from the above theory that a "proposition," in the sense in which a proposition is supposed to be the object of a judgment, is a false abstraction, because a judgment has several objects, not one. It is the severalness of the objects in judgment (as opposed to perception) which has led people to speak of thought as "discursive," though they do not appear to have realized clearly what was meant by this epithet.

Owing to the plurality of the objects of a single judgment, it follows that what we call a "proposition" (in the sense in which this is distinguished from the phrase expressing it) is not a single entity at all. That is to say, the phrase which expresses a proposition is what we call an "incomplete" symbol[16]; it does not have meaning in itself, but requires some supplementation in order to acquire a complete meaning. This fact is somewhat concealed by the circumstance that judgment in itself supplies a sufficient supplement, and that judgment in itself makes no verbal addition to the proposition. Thus "the proposition 'Socrates is human'" uses "Socrates is human" in a way which requires a supplement of some kind before it acquires a complete meaning; but when I judge "Socrates is human," the meaning is completed by the act of judging, and we no longer have an incomplete symbol. The fact that propositions are "incomplete symbols" is important philosophically, and is relevant at certain points in symbolic logic.

The judgments we have been dealing with hitherto are such as are of the same form as judgments of perception, i.e. their subjects are always particular and definite. But there are many judgments which are not of this form. Such are "all men are mortal," "I met a man," "some men are Greeks." Before dealing with such judgments, we will introduce some technical terms.

We will give the name of "a complex" to any such object as " in the relation to " or " having the quality ," or " and and standing in the relation ." Broadly speaking, a complex is anything which occurs in the universe and is not simple. We will call a judgment elementary when it merely asserts such things as " has the relation to ," " has the quality " or " and and stand in the relation ." Then an elementary judgment is true when there is a corresponding complex, and false when there is no corresponding complex.

But take now such a proposition as "all men are mortal." Here the judgment does not correspond to one complex, but to many, namely "Socrates is mortal," "Plato is mortal," "Aristotle is mortal," etc. (For the moment, it is unnecessary to inquire whether each of these does not require further treatment before we reach the ultimate complexes involved. For purposes of illustration, "Socrates is mortal" is here treated as an elementary judgment, though it is in fact not one, as will be explained later. Truly elementary judgments are not very easily found.) We do not mean to deny that there may be some relation of the concept man to the concept mortal which may be equivalent to "all men are mortal," but in any case this relation is not the same thing as what we affirm when we say that all men are mortal. Our judgment that all men are mortal collects together a number of elementary judgments. It is not, however, composed of these, since (e.g.) the fact that Socrates is mortal is no part of what we assert, as may be seen by considering the fact that our assertion can be understood by a person who has never heard of Socrates. In order to understand the judgment "all men are mortal," it is not necessary to know what men there are. We must admit, therefore, as a radically new kind of judgment, such general assertions as "all men are mortal." We assert that, given that is human, is always mortal. That is, we assert " is mortal" of every which is human.

Thus we are able to judge (whether truly or falsely) that all the objects which have some assigned property also have some other assigned property. That is, given any propositional functions and , there is a judgment asserting with every for which we have . Such judgments we will call general judgments.

It is evident (as explained above) that the definition of truth is different in the case of general judgments from what it was in the case of elementary judgments. Let us call the meaning of truth which we gave for elementary judgments "elementary truth." Then when we assert that it is true that all men are mortal, we shall mean that all judgments of the form " is mortal," where is a man, have elementary truth. We may define this as "truth of the second order" or "second-order truth." Then if we express the proposition "all men are mortal" in the form and call this judgment , then " is true" must be taken to mean " has second-order truth," which in turn means

In order to avoid the necessity for stating explicitly the limitation to which our variable is subject, it is convenient to replace the above interpretation of "all men are mortal" by a slightly different interpretation. The proposition "all men are mortal" is equivalent to "' is a man' implies ' is mortal,' with all possible values of ." Here is not restricted to such values as are men, but may have any value with which "' is a man' implies ' is mortal'" is significant, i.e. either true or false. Such a proposition is called a "formal implication." The advantage of this form is that the values which the variable may take are given by the function to which it is the argument: the values which the variable may take are all those with which the function is significant.

We use the symbol "(" to express the general judgment which asserts all judgments of the form "." Then the judgment "all men are mortal" is equivalent to i.e. (in virtue of the definition of implication) to As we have just seen, the meaning of truth which is applicable to this proposition is not the same as the meaning of truth which is applicable to " is a man" or to " is mortal." And generally, in any judgment (, the sense in which this judgment is or may be true is not the same as that in which is or may be true. If is an elementary judgment, it is true when it points to a corresponding complex. But ( does not point to a single corresponding complex: the corresponding complexes are as numerous as the possible values of .

It follows from the above that such a proposition as "all the judgments made by Epimenides are true" will only be prima facie capable of truth if all his judgments are of the same order. If they are of varying orders, of which the nth is the highest, we may make n assertions of the form "all the judgments of order made by Epimenides are true," where has all values up to . But no such judgment can include itself in its own scope, since such a judgment is always of higher order than the judgments to which it refers.

Let us consider next what is meant by the negation of a proposition of the form "." We observe, to begin with, that " in some cases," or " sometimes," is a judgment which is on a par with " in all cases," or " always." The judgment " sometimes" is true if one or more values of exist for which is true. We will express the proposition " sometimes" by the notation "," where "" stands for "there exists," and the whole symbol may be read "there exists an such that ." We take the two kinds of judgment expressed by "" and "" as primitive ideas. We also take as a primitive idea the negation of an elementary proposition. We can then define the negations of and (. The negation of any proposition will be denoted by the symbol "." Then the negation of will be defined as meaning and the negation of will be defined as meaning "." Thus, in the traditional language of formal logic, the negation of a universal affirmative is to be defined as the particular negative, and the negation of the particular affirmative is to be defined as the universal negative. Hence the meaning of negation for such propositions is different from the meaning of negation for elementary propositions.

An analogous explanation will apply to disjunction. Consider the statement "either , or always." We will denote the disjunction of two propositions , by "." Then our statement is "." We will suppose that is an elementary proposition, and that is always an elementary proposition. We take the disjunction of two elementary propositions as a primitive idea, and we wish to define the disjunction This may be defined as "," i.e. "either is true, or is always true" is to mean "' or ' is always true." Similarly we will define as meaning "," i.e. we define "either is true or there is an for which is true" as meaning "there is an for which either or is true." Similarly we can define a disjunction of two universal propositions: "" will be defined as meaning "," i.e. "either is always true or is always true" is to mean "' or ' is always true." By this method we obtain definitions of disjunctions containing propositions of the form or ( in terms of disjunctions of elementary propositions; but the meaning of "disjunction" is not the same for propositions of the forms (, (, as it was for elementary propositions.

The considerations so far adduced in favour of the view that a function cannot significantly have as argument anything defined in terms of the function itself have been more or less indirect. But a direct consideration of the kinds of functions which have functions as arguments and the kinds of functions which have arguments other than functions will show, if we are not mistaken, that not only is it impossible for a function to have itself or anything derived from it as argument, but that, if is another function such that there are arguments with which both "" and "" are significant, then and anything derived from it cannot significantly be argument to . This arises from the fact that a function is essentially an ambiguity, and that, if it is to occur in a definite proposition, it must occur in such a way that the ambiguity has disappeared, and a wholly unambiguous statement has resulted. A few illustrations will make this clear. Thus "," which we have already considered, is a function of ; as soon as is assigned, we have a definite proposition, wholly free from ambiguity. But it is obvious that we cannot substitute for the function something which is not a function: "" means " in all cases," and depends for its significance upon the fact that there are "cases" of , i.e. upon the ambiguity which is characteristic of a function. This instance illustrates the fact that, when a function can occur significantly as argument, something which is not a function cannot occur significantly as argument.

But conversely, when something which is not a function can occur significantly as argument, a function cannot occur significantly. Take, e.g. " is a man," and consider " is a man." Here there is nothing to eliminate the ambiguity which constitutes ; there is thus nothing definite which is said to be a man. A function, in fact, is not a definite object, which could be or not be a man; it is a mere ambiguity awaiting determination, and in order that it may occur significantly it must receive the necessary determination, which it obviously does not receive if it is merely substituted for something determinate in a proposition[17]. This argument does not, however, apply directly as against such a statement as " is a man." Common sense would pronounce such a statement to be meaningless, but it cannot be condemned on the ground of ambiguity in its subject. We need here a new objection, namely the following: A proposition is not a single entity, but a relation of several; hence a statement in which a proposition appears as subject will only be significant if it can be reduced to a statement about the terms which appear in the proposition. A proposition, like such phrases as "the so-and-so," where grammatically it appears as subject, must be broken up into its constituents if we are to find the true subject or subjects[18]. But in such a statement as " is a man," where is a proposition, this is not possible. Hence " is a man" is meaningless.

We are thus led to the conclusion, both from the vicious-circle principle and from direct inspection, that the functions to which a given object can be an argument are incapable of being arguments to each other, and that they have no term in common with the functions to which they can be arguments. We are thus led to construct a hierarchy. Beginning with and the other terms which can be arguments to the same functions to which can be argument, we come next to functions to which is a possible argument, and then to functions to which such functions are possible arguments, and so on. But the hierarchy which has to be constructed is not so simple as might at first appear. The functions which can take as argument form an illegitimate totality, and themselves require division into a hierarchy of functions. This is easily seen as follows. Let be a function of the two variables and . Then if, keeping fixed for the moment, we assert this with all possible values of , we obtain a proposition: Here, if is variable, we have a function of ; but as this function involves a totality of values of [19], it cannot itself be one of the values included in the totality, by the vicious-circle principle. It follows that the totality of values of concerned in ( is not the totality of all functions in which can occur as argument, and that there is no such totality as that of all functions in which can occur as argument.