COSMOLOGY

The term cosmology stands for a family of related inquiries, all in some sense concerned with the world at large. Two main subgroups of uses may be distinguished: those belonging to philosophy and those belonging to science.

"Cosmology" has received wide currency as a name for a branch of metaphysics, ever since Christian von Wolff, in his Discursus Praeliminaris de Philosophia in Genere (1728), gave cosmology a prominent place in his classificatory scheme of the main forms of philosophical knowledge and distinguished this branch from ontology, theology, and psychology. (See Discourse on Philosophy in General, translated by R. J. Blackwell, Indianapolis, 1963, Para. 77). Despite the severe strictures that Immanuel Kant leveled against the pursuit of rational cosmology in his Critique of Pure Reason, the term has continued to enjoy a standard use among many philosophers. For example, it occupies a central place in the manuals of scholastic philosophy; these adhere, for the most part, to the Wolffian scheme of classification of the branches of metaphysics. The term has been used, too, by many philosophers not in the scholastic tradition; for example, A. E. Taylor in his Elements of Metaphysics (London, 1903) assigns to cosmology the task of considering "the meaning and validity of the most universal conceptions of which we seek to understand the nature of the individual objects which make up the experienced physical world, 'extension,' 'succession,' 'space,' 'time,' 'number,' 'magnitude,' 'motion,' 'change,' 'quality,' and the more complex categories of 'matter,' 'force,' 'causality,' 'interaction,' 'thinghood,' and so forth" (p. 43). Cosmology is sometimes understood even more broadly, as being synonymous with speculative philosophy in its most comprehensive sense. Thus in Alfred North Whitehead's Process and Reality (New York, 1929), whose subtitle is "An Essay in Cosmology," the attempt is made to construct a categorial scheme of general ideas "in terms of which every element of our experience can be interpreted" (p. 4).

In its second major use, the term cosmology designates a science in which the joint efforts of the observational astronomer and the theoretical physicist are devoted to giving an account of the large-scale properties of the astronomical or physical universe as a whole. The task of constructing models of the universe that are suggested by and tested by appeals to the observational findings of the astronomer distinguishes the enterprise of scientific cosmology from the a priori investigations of rational cosmology (as a branch of metaphysics) and the purely conceptual and categorial analyses of the speculative philosopher. Nevertheless, even scientific cosmology poses a number of philosophical questions. The sum of these—and they are principally methodological and epistemological in character—constitutes the philosophy of scientific cosmology. The present entry is concerned with the philosophy of cosmology in this sense. Attention will be focused on a central theme in this area: the question whether cosmology must employ a method different from that employed in other empirical sciences because of its distinctive subject matter, namely, the universe as a unique system.

Description or Explanation?

Is the familiar distinction between description and explanation (or the corresponding one drawn between sciences still in the early stages that are primarily descriptive, and those that have progressed to the predominating use of the explanatory aspects of theory) a distinction that can be profitably applied in giving an account of the logic of cosmology? No simple and unqualified answer can be given. For, on the one hand, cosmology, in attempting to gain knowledge of the universe as a whole, certainly is not content to rest with the observational reports of the astronomer, and therefore cannot be classed with the descriptive sciences. On the other hand, in advancing to the level of theory, as cosmology in a qualified sense certainly does, it is not primarily concerned with the explanation of laws—as is the case with other explanatory sciences.

If by description is meant giving an account of some single event or object in observational terms, or (in an extended sense of "description") formulating a generalization (law) in observational terms which refers to the observable or measurable properties and relations of a class of events, then cosmology, which is interested in giving an account of the universe as a whole, is not engaged in description. Even if we recognize, as we must, the descriptive activities of observational cosmology as a branch of observational astronomy, these fall short of giving us an adequate account of the universe as a whole. All that astronomy can give us is a description of the domain of objects and events within the range of its most powerful instruments. At the present time, however, these instruments, have not reached the limit, if there is a limit, to what is in principle observable. Moreover, even if the universe were in some sense finite and wholly explorable by actually or theoretically available instruments, the statement that what is thus observationally explored is in fact the universe as a whole would not be warranted by observational evidence alone. Such a statement could not, therefore, be part of the description of the universe, insofar as this description is a report of what is found. The claim that the universe is open to complete inspection requires the support of theory. It is a statement which is not included in the description, but is a rider to the description—to the effect that the description as given is of the universe as complete; considerations other than purely observational ones are needed to support this claim.

If cosmology is not content with description, does it then aim at giving explanations? Here our answer must be qualified. In the case of ordinary empirical generalizations, where there are multiple instances of some phenomenon of which we have examined a limited number, we say that the law supported by this evidence may be used as a reliable rule of inference. Since the law applies to a kind of subject matter, or a type of phenomenon, it can be upheld as a useful means for predicting and explaining those instances that can be brought within its scope. But in cosmology, the primary goal is not to establish laws. The universe, by definition, is a unique object or system. Cosmology does not undertake to establish laws about universes; at best one can establish laws about the constituents of the universe. The relation that the observable portion of the universe bears to the universe is that of part to presumed whole, rather than that of instance to law. Hence, if to explain means to bring an instance under a law, this mode of explanation, which is a characteristic concern of other branches of physical science, does not characterize cosmology.

Can it be said, then, cosmology aims at giving explanations in the sense in which theories are employed to explain laws? Here our answer, once more, cannot be a straightforward "yes" or "no." The characteristic device employed by theoretical cosmology is a model of the universe, and a model in many respects functions precisely as a theory does. It is a conceptual construction that cannot be said to be a mere report of what is already found in observation, nor even an anticipated description of what might be found in future observations. Rather, it is a means for making the observational data themselves intelligible. However, the facts that the cosmologist wants to explain are not laws in the ordinary sense of the term, and so in this respect the purpose of a model of the universe is not identical with that of a theory in the ordinary sense.

Consider, for example, the question "Why is the apparent magnitude of galaxies correlated with their red shift?" This question asks for an explanation of an important datum of observation. The observed fact is sometimes called Hubble's "law," but it is a law only in the peculiar sense in which we refer to Kepler's laws as "laws." That is, Hubble's law tells us something about a particular distribution or process of a unique set of objects, namely the system of galaxies, just as Kepler's laws tell us something about the orbits of the planets in our solar system, not in any solar system. In general, however, laws of science are characterized by their universal form. They are unrestricted in scope and are not ostensibly tied to objects or events specifically located in some particular space-time region. Thus Newton's law of gravitation, for example, says that for any two bodies, the gravitational force that holds between them is inversely as the square of their distance apart and proportional to their masses. Now when we deal with the system of bodies and processes that constitute the unique configuration we call the universe, we are not dealing with any configuration of events and objects; we are dealing with the configuration actually observed and given.

An interesting and important question that can be raised here, however, is whether the unrestricted laws of ordinary physics are not themselves, in a more profound sense, relatively restricted, since they apply to bodies or phenomena within the ultimately unique configuration that constitutes the physical universe. From this point of view, the study of cosmology sets the environment and limiting framework for all other branches of physical science. Hence it is not unreasonable to expect—as E. A. Milne, D. W. Sciama, and others have pointed out—that one may hope to understand the laws of physics themselves in terms of the unique background making up the universe studied in cosmology. Such a claim, however, is associated only with certain specific models, namely, the kinematic model as worked out by Milne and the steady state model as sketched by Hermann Bondi and Sciama, and therefore this idea of explaining all laws by a cosmological model cannot be held up as a working goal for all cosmological models. In fact, the majority of models developed within the framework of general relativity theory have not been designed to embody this feature.

Observation and Theory

The study of cosmology has two lines of attack, that of the observational astronomer and that of the theoretical physicist. One might say that both the observational astronomer and the theoretical cosmologist are studying the universe, though from different vantage points, or that one supplies observational data about the universe that the other undertakes to interpret; but this is, at best, only a sketch of the situation and is in some ways seriously misleading. For it will not do to say that both the astronomer and the theoretical cosmologist are studying the universe, as if the universe is laid out for identification before them and the only difference between them is in approach and method. If we look more closely at the study of cosmology, the situation is rather different.

The observational astronomer is not confronted with the universe as an observationally complete whole. Instead, he obtains observational clues from various instruments about a large population of identifiable subsystems—namely, individual galaxies and clusters of galaxies. This population of observable entities is sometimes referred to as "the observable universe." However, this phrase is not to be understood in the sense that we have independent means for identifying the universe and that we wish to refer to it insofar as it is being observed. "The observable universe" is not the same as "the universe observed." What the astronomer reports on of relevance to cosmology is an observable population of galaxies and clusters of galaxies. These observational reports have to do with such matters as the spatial distribution of galaxies, their systematic motions, density, spectroscopic patterns, individual shapes, and stellar composition.

The population of subsystems that makes up the observable universe is now, as in a sense it must always be, a finite population. With the advance in the power and sensitivity of instruments, knowledge of the extent of this population and the refinement in the details of the reports about this population are improved. Although it is regarded as likely that further advances in observational resources will disclose a wider population of subsystems similar to those already observed, it must be remembered that it is always possible for further observations to disclose as basic constituents of the universe astronomical units of a higher degree of inclusiveness than galaxies or clusters of galaxies, or even entities of an altogether different type from those heretofore disclosed. Whatever may be the case in the future, it certainly is the case at present that what comes within the observational reaches of the astronomer is definitely not the universe as an absolute whole, if there is in fact such a whole.

When we say that the theoretical cosmologist studies the universe in order to understand it or make it intelligible, what is it that he studies? He does not study the universe in any direct way, if that means having before him a readily identified object which he tries to comprehend, for example by subsuming it under some law. Nor, as we have just seen, does the universe he studies consist of a complete population of entities about which the observational astronomer furnishes him detailed reports. The theoretical cosmologist is not given information about the universe as a whole, nor even about what lies beyond the immediate range of the astronomer's instruments.

What then does he study? A brief and simple answer is to say that he constructs a model of the universe and that he studies the way in which this model may be used to interpret the observational data already available. The cosmologist will use his model to interpret the data assembled by the observational astronomer and to guide the astronomer in the search for further data. Insofar as the use of theoretical models proves satisfactory, we may say that cosmology has helped us to understand the universe and to make it intelligible. This is not to be understood, however, as meaning that even at the end of a relatively successful course of inquiry, the cosmologist has been able to confront the universe directly as some kind of readily identifiable object, system, or class of objects. What is to be understood by "the universe," in short, can only be approached and identified through the use of models, not independently of them.

The Model and Its Construction

The kind of model that the cosmologist constructs is wholly conceptual rather than material. It consists of different sorts of symbols including ordinary language, mathematical language, diagrams, and charts, all of which will normally be employed in presenting a given model. A model of the universe is not something that can be directly visualized or completely represented in a pictorial diagram. Consider, for example, a typical model in which use is made of a geometric mode of representation according to which the galaxies are treated as a set of mathematical points that trace out a set of geodesic curves in space-time. In this case, the metric of this set of points is given by the general Robertson-Walker expression for the space-time interval (ds ):

where R (t ) is the expansion factor, k is a constant whose value determines whether space is Euclidean or non-Euclidean, c is the velocity of light, and r, θ, and ϕ are spatial coordinates. In addition to the specification of purely geometric or kinematic features, which are specified by introducing appropriate values for the curvature constant (k ) and the expansion factor (R (t )), a model will also require some assignment of specific dynamic or gravitational properties to the entities thus represented. Additional formulas will then be required, and these will normally involve relativity theory or some equivalent branch of physics. It is clear that however much a simple diagram making use of dots and lines may serve to give us a visual representation of what we are talking about, this hardly suffices to encompass all those additional features of the model not included in the diagram.

Although the cosmologist cannot inspect the original, the universe itself, he nevertheless undertakes to make a model of it. How is this done? The answer is to be found by noting the various clues and sources to which the cosmologist appeals in determining the properties to be assigned to his model. These are of two principal types: observational clues provided by the astronomer, and theoretical principles thought to be of relevance to the cosmologic problem.

observational clues

In general, the observational data the astronomer gathers aid the cosmologist by suggesting ways of assigning certain idealized properties to the model, by providing empirically ascertained values for the constants and variables in the model, and by offering tests for the adequacy of the model as a tool for predicting observable matters of fact.

Idealized properties

The kinds of entities and their properties that the astronomer observes suggest to the cosmologist the lines to follow in developing a simplified and idealized conception of the universe. Let us take some examples. The galaxies, though of enormous physical bulk, may be considered for purposes of the model as particles making up a continuous and perfect fluid. The advantage of treating the galaxies in this fashion is that it permits a great simplification of the problem, to which readily available mathematical tools of representation and calculation may be applied. Here, of course, the cosmologist adopts a technique that is universally adopted in other branches of physical science and with similar justification. If necessary, suitable corrections to this idealization can always be introduced when application is made of the model to "describe" the actual universe.

An important feature of the domain of galaxies already observed is their spatial distribution. The actual spatial distribution of the galaxies is roughly homogeneous and isotropic when fairly large volumes of space are considered. On a smaller scale, departures from homogeneity become more noticeable, in the clustering of the galaxies, for example. When still smaller volumes of space are investigated, homogeneity breaks down altogether. In general, then, the claim to the uniformity of distribution of galaxies can be upheld only if one takes a sufficiently large unit of volume, say 3.5 × 10⁸ parsecs in diameter. Yet in constructing his model, and as a first approximation, the cosmologist will assign a complete homogeneity to his model of the universe. The expression "cosmological principle" is commonly used to designate this feature of spatial homogeneity. Models that satisfy this cosmological principle, and thus possess the feature of spatial homogeneity, are known as uniform model universes.

When put into mathematical language, a uniform model universe is one possessing a constant curvature at a given moment of time. In the language of general relativity theory, since the density and pressure of material that make up the model are the same in all volumes of space at a given time, whatever their size, a geometric representation of this fact will involve the use of one or another of the spaces of constant curvature. All segments of space of the entire universe will have the same curvature. Such a model clearly requires a process of idealizing and simplifying the spatial distribution of bodies actually observed. For if we were to use the language of geometry to describe the actually observed spatial distribution, we should have to note the actual local departures from homogeneity or constancy of curvature.

In constructing a model of the universe that embodies the feature of spatial homogeneity or constancy of curvature, it is not enough to specify what that curvature is at the present moment of cosmic time. A fully determined model requires (in addition to other features) that the spatial properties of the universe be specified for any point in its past or future. Here there are, broadly speaking, two possibilities. According to one, the spatial properties of the universe remain the same at all times; this view is upheld by those who adhere to the "perfect cosmological principle" and use it to define the properties of the steady state model. A second alternative is to adopt the cosmological principle in its more restricted form as designating merely spatial uniformity, as is the case with the orthodox cosmological models of general relativity. For such models, the entire history of the universe, from a spatial point of view, could be specified if one knew just one thing—the rate at which the distance between any two galaxies changes with time. In a universe characterized simply by the cosmological principle, since an observer would always find a spatially isotropic distribution of particles about him, the only basic feature subject to change is a temporally noticeable feature, namely, changes in the density of the distribution of particles. Such changes in the density might then serve to define a cosmic "clock."

Empirically obtained values

A second important function that the appeal to observational data serves in the construction of cosmological models is that of yielding empirically obtained values for some of the constants and variables of theory. For example, in relativistic uniform models of the expanding universe, the defining characteristics of a particular model need to be specified by assigning values for the following quantities: the cosmological constant (λ ), the temporal pattern of the universe as determined by evaluating the function R (t ), the values for the velocity factor and the acceleration factor in the velocity-distance relation that specify how the galaxies are moving, the density (ρ ) and the pressure (p ) of the material and energetic content that fills the universe, and the curvature constant (k ). Observational evidence is, at present, either not available at all or not accurate enough to give sufficiently precise determinations for all of these terms. The cosmologist must, therefore, use whatever data is available to eliminate those models that are incompatible with present observations and to suggest lines of inquiry that will help to narrow the field down to those models that can be further tested by observation.

One overall condition for the acceptance of a model is, of course, the consistency of the empirically obtained values it proposes. In a particular model, a particular combination of empirically assigned curvature and density values, for example, may lead to a calculated "age of the universe" that will be inconsistent with an independently obtained estimate for the lower bound of such an "age"; the estimated time scale of the universe will be too short. In general what is sought is a model all of whose empirically ascertainable values are mutually consistent within the available limits of accuracy.

Empirical tests

Finally, as a natural extension of the point just made, we see how the data obtained by the astronomer serve to test the calculated numerical values for quantities appearing in the cosmologist's equations or other qualitative predictions made on the basis of a given model. Thus whether the extremely remote galaxies at the horizon of the now observable population of galaxies have roughly the same characteristics as those that are nearer is an important question much discussed at the present time as a means for evaluating the rival claims of the steady state and evolutionary theories. The steady state theory claims that galaxies that are at the outer limits of observability should have roughly the same characteristics as those at lesser distances. According to various "evolutionary" models, those same remote galaxies, from which we receive light and other forms of radiation emitted billions of years ago, could, in effect, tell us about the earlier stages of the evolution of the universe. Since conditions at the time of emission were presumably different from what they are now, these very remote galaxies should display differences from those that are nearer to us in at least some of their properties, and these differences should give us valuable clues about the course of development of the universe as a whole. In this regard a number of delicate questions that are the subject of much controversy have arisen in current research.

theoretical ideas

A second major source of ideas in the construction of cosmological models is to be found in the conceptual resources of mathematical physics. Here there are two broad possibilities that confront the cosmologist.

Use of established principles

As a first possibility, the cosmologist may turn to some already established body of physical theory as expressed in fundamental principles and derived laws. Such theory will normally have already been found to be successful in dealing with a variety of physical problems of lesser scope than, and wide differences from, the purely cosmological problem. The cosmologist will nevertheless propose to see to what extent the same general body of ideas may be used when applied to the distinctive subject matter of cosmology. He will investigate to what extent the universe as a physical system has a detailed structure that may be articulated and specified by means of the selected physical theory. For example, he may use Newtonian mechanics to construct a model of the universe. Isaac Newton himself drew, in a general qualitative way, the cosmological consequences of using the inverse square law of gravitation as a guide. He argued that the universe, throughout its infinite space, must be filled by a more or less evenly distributed matter. For if all the matter that exists were to be confined to a finite "island" in an infinite "ocean" of space, it would have a center of mass toward which, in time, all matter would move by gravitational attraction. The fact is that no such motion is found, and Newton concluded, therefore, that matter is distributed uniformly throughout an infinite space.

At the present time, the primary and predominant source to which the cosmologist turns is the general theory of relativity as expressed in Albert Einstein's general field equations. These equations specify the relations between the space-time metric of any physical domain and its material or energetic content. The discovery of solutions to those field equations that are of special relevance to the cosmologic situation has led to the construction of several varieties of relativistic models. The other major use to which the field equations of general relativity have been put takes the form of the Schwarzschild solution. It was this solution that first afforded the opportunity for testing the predictive and explanatory powers of the theory as a whole. Karl Schwarzschild's solution is particularly applicable to a physical system such as we encounter in the solar system, namely, a single massive particle (the sun) in whose neighborhood we may study the behavior of much smaller masses (the planets) and light rays. The success of its predictions and explanations has been the primary basis for the confidence placed in the general theory.

To return then to cosmology: Within the broad class of homogeneous, or uniform, model universes we may distinguish the nonstatic models and the static models. Among the static models is Einstein's original model of 1917, which pictured the universe as finite and unbounded; in the light of the subsequent discovery of the mutual recession of the galaxies, it is no longer considered adequate. The nonstatic models include the ever-expanding-universe models that originate from zero or in some finite volume, and oscillating models that undergo alternate contractions and expansions. Within each of these groups, individuating characteristics for a particular model are to be found in the choice of values for the curvature, age, density, and cosmological constant. No single model has as yet been universally adopted.

Creation of new principles

The other broad possibility for furnishing theoretical ideas for cosmologic models is one in which the cosmologist, instead of appealing to already established principles or laws, for example those of relativistic mechanics, undertakes to create afresh basic principles thought to be of special relevance to the cosmologic problem. By way of illustration, there is the conflict of the 1930s and 1940s between the way in which E. A. Milne sought to establish his kinematic model and the more orthodox procedures of relativistic cosmology. Although Milne did use the formulas of special relativity, he did not take these over directly from Einstein's own presentation; Milne attempted instead to derive them from what he thought of as more basic and primitive postulates. These postulates, he claimed, state the conditions for the measurement of time and for the communication of results by different timekeepers and observers.

A more recent example of the same sort of procedure is the steady state model of the universe proposed by Bondi and Thomas Gold in 1948. In support of this model, it is argued that since the universe is unique, there is no reason to believe that the laws which apply to smaller-scale physical phenomena, for example in laboratory terrestrial physics, or even in the domain of gravitational phenomena in the solar system, need be expected to apply to the universe as a whole. Therefore, instead of taking such laws as the point of departure in investigating the physical properties of the universe as a whole, it is suggested that the cosmologist can actually enjoy a far greater freedom than is believed possible in orthodox relativistic cosmologies. Let the cosmologist adopt any "laws" or principles which he believes are appropriate to the study of the universe as a whole, even though these may not have been established or confirmed in other (smaller-scale) areas of physical phenomena. The important thing is to see whether using these laws and principles leads to confirmable empirical results and whether they help to increase and deepen our understanding of the universe.

Those who favor this view (Bondi and Gold among others) determine some of the major features of the steady state model by appeal to the specially introduced postulate known as the perfect cosmological principle. This principle was not in prior use in other branches of physics but was introduced because of its special relevance to cosmology. (Fred Hoyle's model of the steady state universe proceeds along more conventional lines, at least in this respect. Although it differs from the expanding universe models of general relativity in abandoning the principle of the conservation of matter—in order to make possible the idea of the continuous creation of matter—it appeals for its basic physical principles, although in modified form, to the field equations of general relativity.)

One general motive that seems to inspire the setting up of specially devised principles for cosmologic models is the desire to show that the science of cosmology is basic to all other physical sciences. Instead of appealing to other branches of physics for principles to be used in describing the features of the universe as a whole, it is thought desirable that one should be able, eventually, to show that the laws of ordinary physics can be linked with the properties of the universe as a whole. The universe would then disclose itself to be a unitary physical system within which it would be possible in principle to deduce ordinary physical laws from the principles of cosmology. Milne undertook to show how, for example, the inverse square law of gravitation, among other things, could be deduced from such more fundamental cosmological ideas. Similarly, within the framework of a steady state model, Sciama attempts to show how the local inertial properties of matter can be linked (as Ernst Mach originally proposed) with the distribution of masses in the universe at large.

From a logical point of view, there is no reason to discourage such efforts. On the contrary, the realization of such a goal would be of immeasurable significance for all of science, and one should in logic suspend judgment until such a program can be carried through with some fair degree of success.

Meanwhile, it is necessary to point out that some of the writers who favor this approach put methodological interpretations on the use and warrant for the specially devised principles that are not acceptable, whatever the eventual success or promise of the program as a whole. Thus Milne and Bondi, who support different models, are each concerned to stress what they take to be a special method for cosmology—as contrasted with other branches of physics. Milne, for example, thought of ordinary physics as employing an inductive method, whereas cosmology, he believed, should be based on a deductive method. Cosmology, he argued, should not employ the laws of ordinary physics to the extent that these are inductively warranted. This was his major complaint against what he took to be the faulty procedure of relativistic cosmologies founded on the "inductively established" principles of general relativity. In making this claim, Milne was in error, since the principles of general relativity theory are not, as he thought, ordinary inductive generalizations.

In fact, Milne's own appeal to "self-evidence" as the warrant for introducing his preferred cosmological principles must be rejected, for the appeal is groundless and fails to support the certainty and uniqueness which he claimed for his principles. In constructing a model of the universe, the cosmologist is engaged in setting up a theoretical tool for dealing with the facts of observation. Whether he gets his theoretical principles by "borrowing" them from some other branch of physics or whether he creates them especially for the problem at hand is of secondary importance to what he does with these principles once he has them and how he evaluates the results he achieves. There is a common method that characterizes cosmology regardless of the particular model being proposed or favored, and it is precisely the same method which is employed in other branches of physics. Moreover, the same criteria of evaluation need to be brought to bear in the appraisal of results in cosmology as in other areas of science. Far from including any appeal to self-evidence or to similar rationalistic demands, a satisfactory model requires the constant support provided by observational evidence.

Cognitive Worth of Models

Consideration of the goals set by scientific cosmology gives rise to a central philosophical question, that of determining the cognitive worth of any cosmological model. This is an epistemological question and may be put in terms of the traditional issues separating the realist and the conceptualist (or the instrumentalist). Should we say with the realist that cosmological models offer us an account of the structure of the independently existing universe, or, rather, should we say with the conceptualist that these models are simply useful means of presenting and interpreting observational data?

As a basis for clarifying the issue at hand, it will be helpful to point out a fundamental ambiguity in the use of the term universe itself. Employed without the qualifying adjectives "observed" or "observable," it may have at least two quite distinct senses. One meaning of universe is "that to which the observed universe belongs"; another is "that which is characterized by a cosmological model." So far as the realist is concerned, the two meanings are equivalent; in his view the universe defined by a cosmological model will be the same universe as the one described by the expression "that to which the observed universe belongs." But the realist's view of cosmological models cannot be assumed in advance to be the only tenable one. Thus the distinction suggested here has the merit of permitting us to keep this question open. If later a realist philosophy is accepted, the appropriate modifications can be made. Clearly, we do not need to commit ourselves to the position that everything properly said of "the universe" in the sense of "that which is characterized by a cosmological model" can also be said of "the universe" in the sense of "that to which the observed universe belongs." For example, we might want to attribute the property of being "a whole" or "an absolute totality" to the universe as characterized by a particular model, but not to the universe in the sense of that to which the observed universe belongs.

Cosmology aims at articulating the character of the universe as a whole. To that extent, then, it rests on the methodological postulate that the universe is a whole. The specific character of the whole will, of course, be variously described by various models. What remains fixed, however, is the assumption that the goal of cosmology is to characterize the universe as a whole. Therefore the statement "The universe is a whole" is in this context an analytic statement, a matter of definition. But note that it is a definition in which "the universe" is used to signify "that which is characterized by a cosmological model." Not only does cosmology require that, as a matter of definition, the universe be thought of as a whole (in the sense of being intelligible in the way that mathematical classes, geometrical relations, or physical systems are); it also postulates that the universe as a whole is unique or absolute. This means that there is just one such class, pattern, or system, and that all other physical processes or systems of lesser duration or spatial extent are to be taken only as parts of this all-embracing whole. Since each model will so define the universe, it would be a misuse of language to speak of a plurality of universes. Again, of course, the precise structure of this unique or absolute whole will, at least in some respects, vary from model to model.

But what if "the universe" means "that to which the observed universe belongs"? Is the statement "The universe is an absolute, unique whole" still analytic? To this the answer must be no. For when we use "the universe" in this sense, we move from methodology to ontology. In contrast to the case of the universe as defined by a cosmological model, we are no longer committed by the basic methodological postulate of cosmology to saying that the universe is a whole. True, in setting up a science, it may be necessary to presuppose the existence of some pervasive structure as the object of study. Yet such a presupposition need not be binding on what the universe is existentially. So long as "the universe" means simply "that to which the observed universe belongs," nothing in this meaning contains analytically the notion of its being a "whole" or an "absolute whole." Indeed, even if we grant that the observed universe is structured in some manner, this does not entail that the wider universe of which it is a part is also pervasively structured. Nor does the fact that we describe the observed universe as "part of" or "that which belongs to" something else require us to say that the universe to which it belongs is a unique or absolute whole. For our reliance on such terms as part, whole, and belong reveals merely that the mind, in reaching into the unfamiliar, must use analogies in order to relate the unfamiliar to what it already knows.

The universe as the "something more" than the observed universe may well be a complete, unique and intelligibly structured whole. But the claim that we are able to say so is something to which we need not commit ourselves. It is better left as an open question, since, strictly speaking, it is one on which we neither have nor can have any knowledge. Stipulating an affirmative answer by definition does not, of course, establish such knowledge.

See also Creation and Conservation, Religious Doctrine of; Einstein, Albert; Mach, Ernst; Newton, Isaac; Relativity Theory; Taylor, Alfred Edward; Time; Whitehead, Alfred North; Wolff, Christian.

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