1. What is the "empiricist consensus"?

This term is an artificial label (used for present teaching purposes) to designate a very general position with respect to scientific knowledge. This label is approximately synonymous with the expressions "the received view," "the empiricist view or model," "the "positivist view or model." All these names refer to the view of science then tended to dominate in most (but not all) philosophy of science for roughly the first half of this century. Insofar as those who held it tended to equate scientific knowledge with all knowledge, we may also say it is a very broad outlook on human knowledge in general, in other words it is a general epistemological point of view. This perspective on the nature of knowledge also tended to lead those who held it to accept certain general views with respect to the nature of reality and values. In other words, while the empiricist consensus was primarily an epistemological orientation, it tended also to have metaphysical and axiological dimensions.

2. Who held this view?

It is important to stress that within this general consensus, there was considerable room for diversity among philosophers. Thus one large group of philosophers within this consensus could be labeled positivists, or more specifically "logical positivists" or "logical empiricists," and indeed they tended to dominate, but other philosophers, for example, Bertrand Russell or Karl Popper, could be placed within this consensus but disagreed sharply with the positivists.

3. How has the philosophy of science been shaped by this consensus?

Insofar as they tended to share certain assumptions, debates among philosophers within the consensus could be considered "internal" to that consensus, and these tended to dominate philosophy of science until the sixties. There were always critics who stood outside the dominant consensus (e.g. Michael Polanyi, Pierre Duhem, Alfred North Whitehead or Alexandre Koyre). However, their disagreement with the dominant consensus could be considered "external" to the main field of debate within philosophy of science, and was thus largely ignored within the consensus. But ultimately disagreements and internal problems within the consensus weakened it to the point where the "dam broke" and the once dominant consensus became flooded with both internal and external problems. This happened to coincide rather closely with the publication of Thomas Kuhn's Structure of Scientific Revolutions, which therefore often has been seen as a primary cause for the dissolution of the consensus. It is certainly an exaggeration to regard Kuhn's book as having "caused" the break up of the empiricist consensus, but as events happened, its time of publication was such that debates in philosophy of science during the breakup of the consensus often took Kuhn's work as a point of departure.

4. What was the view of those who belonged to this consensus?

It must be emphasized that this label is being used to designate an artificial "stereotype" which can be characterized in terms of a number of philosophical views, each one of which could be considered an element of this consensus. All of the elements listed below might not characterize any one real philosopher. However virtually all of those philosophers who clearly belong to this consensus would surely share enough of these elements such that they all could be considered as bearing a very strong philosophical "family resemblance."

5. What are the elements of this consensus?

In the most broad sense the consensus is a position about what one ought to believe regarding the world as experienced by human beings. If one rejects the view that what it is "rational to believe" is "what one ought to believe," one is in effect rejecting a philosophical approach to belief altogether, presumably in favor of some other method, say, flipping a coin or believing everything consistent with some "authority," whether in human or written form. Since philosophers, virtually by definition, will describe as "rational" what one ought to believe, this is a view about what it is rational to believe with respect to the world we experience. Thus one element of the consensus is that it is a theory of "scientific rationality."

6. What does a scientifically rational person believe?

Although "beliefs" may refer to the private psychological states of human beings (thus allowing the possibility of "beliefs" which cannot be expressed in language), for those who held the empiricist consensus, "beliefs" are restricted to what can in some sense be made public (and thus open to analysis) by being communicable in the meaningful expressions, or "sentences" of some language. Insofar as a sentence in a "natural language" (e.g. English of German) can be "ambiguous" or can be given different meanings, we may be justified in believing it under one possible meaning and not justified in believing it under another; we would have in effect two different beliefs. Thus to members of the consensus, those beliefs which one can be justified in believing must be statements expressed by unambiguous sentences of an artificial "ideal" language in which all meaningful expressions can be given precise meanings. As "scientific knowledge" may be considered the entire body of rationally justified beliefs about the world humans experience, another element of the consensus was its "linguistic" view that "science" in effect refers to a very large group of "statements" expressed in a special artificial "language" of precisely defined scientific concepts.

7. What are the possible sources of knowledge about the world experienced by human beings?

Philosophers call statements about the world we experience "synthetic" and distinguish them from those whose subject is really the meanings of terms, which are called "analytic." Some philosophers (often called "rationalists") have held that it is possible to be justified in believing statements which are about the world we experience, even though experience itself never provides adequate evidence for such statements. Such philosophers argued that "Reason" unaided by experience could justify such statements. Statements which can be thus justified independently of experience are called "a priori" and distinguished from those that can be justified only by appeal to experience, "a posteriori" or "empirical" statements. Rationalists thus hold to the possibility of justifying believing "synthetic a priori" statements.

8. What is the position of the empiricist consensus regarding the possibility of synthetic a priori knowledge?

Those who belong to the empiricist consensus commonly reject this possibility. For them all justified belief which is not restricted to the meanings of terms, that is, which "escapes the circle of language" and refers to "the world," is only justified by appeal to experience. In other words, we may say that our rational warrant for holding scientific beliefs must always be a certain body of empirical evidence expressed in certain "observation statement" or "basic statements" which are considered to be directly verified by experience. Thus another element of the consensus was its commitment to empiricism against the rationalist view that some things about the world could be known by pure reasoning. This does not mean that the empiricist consensus has no place for reason in determining scientific belief; far from it, the empiricist consensus strongly holds that also scientific explanation and prediction is made possible through deductive reasoning from both general laws and particular observation statements expressing "initial conditions."

9. Does this mean that we are rational to believe only statements about what we can "observe" (i.e., see, hear, touch, smell or taste)?

A critical problem arises when we reflect on the fact that scientific knowledge most obviously is not prima facie limited to what human beings have observed, in other words scientific beliefs are not limited to what are often called "observation statements." Scientific explanations are often couched in terms of appeals to certain "laws of nature" which are universal statements not about particular observations actually made by human beings but about all or any possible observation which would be made under specified conditions. Such statements may even be about what would be the case under conditions which in fact never occur and possibly never could occur ("subjunctive conditionals" or "counterfactuals"). Furthermore, many sciences are deeply committed to explaining observable phenomena through appeal to general statements about entities, states, or processes, which are postulated to exist but are not in fact observed, and in many cases not even in principle "observable." Whether honored with the label of "law of nature" or not, all such statements which transcend what is, or could ever be, observed, are usually called "theoretical statements." Theoretical statements are marked by the fact that they contain one or more conceptual terms appearing to refer to entities, states, or processes which are not observed. Thus one of the core elements of the empiricist consensus is that one can always distinguish between observational and theoretical statements. Some members of the consensus may have admitted that the observational/theoretical distinction could not be sharply drawn, but all held it to be a real and important distinction.

10. Why is this observational/theoretical distinction so important?

The distinction marks a difference between why we are justified in believing the two kinds of statements. Observation statements (also called "basic statements" or "protocol Statements") are justified by a "direct" appeal to experience; they are the statements of science which "reach outside" the body of language and connect scientific beliefs directly to the world we experience. However, theoretical statements are different; we are not justified in believing them on such direct grounds. These statements are treated as "hypotheses" which we are justified in believing only on the basis of the "evidence" provided by the "observation statements." The observational/theoretical distinction thus marks the distinction between evidence and that which the evidence "supports," what are usually called "theories." Thus theoretical statements are admitted into science only when they can, under specifiable circumstances, be used to deduce directly verifiable "observation statements." A statement about unobservable entities or processes which could not be used to predict and observable consequences is said to have no "empirical content" or "empirical significance" and cannot form an acceptable statement in scientific knowledge.

11. Does the observational evidence prove the theoretical statements for which it is the evidence?

We often talk about the laws of science as having been "proved." The word "prove" indicates that the truth of the evidence, if it is true, establishes necessarily the truth of those statements which such evidence is said to support. It has been widely admitted since the time of Hume that in this sense no empirical evidence about particular observations can ever prove the truth of a universal statement which extends beyond what is already present in the empirical evidence. Holders of the empiricist consensus are all "descendants of Hume" in this respect. Therefore, they can be understood a relaxing the classical demand for certainty (necessarily true) and holding that we are rational to believe theoretical statements which are "well-confirmed" or "corroborated" by the empirical evidence. We express this by saying that another element of the consensus is a commitment to "fallibilism," i.e., the view that no statement in science is ever immune to refutation by some as yet unknown evidence. Fallibilists reject the view that any theory is ever proved.

12. What is the relationship between empirical evidence and theory?

This was the topic over which defenders of the consensus spent most of their energies. The aim of the philosophy of science, so they argued, is to exhibit and analyze this relation, but they disagreed among themselves radically as to the nature of this relation. Originally the hope was that one could "reduce" or translate theoretical statements into open ended sets of observation statements, but gradually this ideal was seen as unattainable. Later on, some, like the positivists Carnap and Reichenbach, were most anxious to develop a "logic of confirmation" according to which one could assess just how probably true any universal statement would be in the light of any particular body of empirical evidence. Others, like Popper, argued that empirical evidence could serve only a negative role, "falsifying" those hypotheses we ought to reject ("falsificationism"). According to this outlook what we ought to believe is those hypotheses subject to rigorous empirical testing which have not been refuted. Nevertheless, both the positivist "verificationist" program and its arch rival within the dominant consensus, the Popperian falsificationist program, both accepted some form of a "deductive-nomological model" (or "DN Model") of scientific explanation and a "hypothetico-deductive model" of the relation between evidence and theoretical statements.

13. What does the "deductive-nomological model" hold regarding the nature of scientific explanations?

According to this model, the aim of science is to provide explanations of particular observed phenomena by showing that the occurrence of such phenomena can be validly deduced from a collection of several theoretical statements of universal laws and other observation statements describing observable "initial" or "antecedent" conditions. When such a deduction is performed prior to the occurrence of the phenomenon, it is called a "prediction," after the phenomenon has already occurred, it is called an "explanation." However the valid deduction of a single observable phenomenon -or even a very large number of phenomena- from a law does not establish the truth of the law. To say a deduction is "valid" means only that if those laws are true then that phenomenon would occur. Laws themselves can be explained by deduction from "higher level" laws, but that, of course, does not establish the truth of those higher level laws.

14. Why are we justified in accepting such scientific explanations?

According to the "hypothetico-deductive model of justification" which came to be an accepted element of the empiricist consensus, when observation reveals that the predicted phenomenon occurs as predicted, the law from which it is predicted is "confirmed," or "corroborated," when observation reveals that phenomena do not occur as predicted, the law is "refuted" or "falsified." Prior to any successful predictions, a theoretical statement is purely "hypothetical." Only direct observation statements may be regarded as "verified." Positivists came to accept that all hypotheses are only "confirmed" by the evidence. Falsificationists emphasized that attempts to refute the hypothesis have failed and spoke of the hypothesis as "corroborated." After it has been confirmed by a wide range of successful predictions, and in the absence of any falsifying failed predictions, theoretical statements move from being of a purely "hypothetical" status to being rationally justified and are often called "laws of nature." The difference between a "law" and a "theoretical hypothesis" (or "conjecture") is thus merely the degree of confirmation or corroboration.

15. How are the universal statements which become accepted as laws related to each other?

Many who belonged to the empiricist consensus held that, at least ideally, lower level theoretical statements (whether regarded as mere "hypotheses" or as "laws") could be explained by showing that they could be deduced from higher level more embracing theoretical statements, until in an ideal perfected science, all phenomena could be seen as deducible from set of relatively few fundamental "laws of nature." The epistemic justification for such theoretical laws, then, is seen as based upon the "foundation" of all actual observation statements which confirm the predictions deduced from the theory; this foundation "supports" or "rationally justifies" the theoretical structure of scientific laws which is erected upon it. Hence this view is said to be a form of "foundationalistic epistemology." One could imagine that in an ideal case the body of rationally justified scientific beliefs, a "total unified science" could be arranged as a hierarchy of statements with the most general all embracing fundamental laws of nature at the top and lower level, more specific laws being deduced from them under restricted conditions, in much the same fashion that the theorems of geometry follow from its fundamental axioms and postulates. However, such a system of deductively interrelated theoretical statements, becomes "science" only because the rational warrant for belief in these statements comes not from the "top down," i.e., from axioms to theorems (as appears to be the case in geometry), but from the "bottom up," i.e., from the empirical observation statements which serve to confirm directly the lowest level most particular statements deduced from the system of theoretical laws.

16. How does this epistemological outlook relate to the "scientific method"?

According to the empiricist consensus what is normally taught as the "scientific method" is in effect those procedures which warrant the rational acceptance of theoretical laws, i.e., the process of deducing empirical consequences from theoretical hypotheses and statements of observable conditions, and the confirmation or refutation of those hypotheses. This procedure is seen as a process of "testing" hypotheses and is regarded as the primary occupation of all science. While the methods for devising the specific tests of individual hypotheses may vary from science to science, all sciences share in common allegiance to this method of testing general laws by empirical evidence as the only method for establishing the rational warrant for accepting theoretical laws and the scientific explanations and predictions deduced from them. Thus defenders of the consensus accepted as another element a commitment to the "unity of science" with respect to its methodology.

17. How do we first arrive at the hypotheses which come to be accepted as laws of nature?
What transforms a hypothesis into an accepted scientific belief is the process of testing by the hypothetico-deductive procedure indicated above. This part of the scientists' activity was often referred to as the "context of justification" and it is often distinguished from the process by which the scientist formulates the hypothesis in the first place, which is called the "context of discovery." Since only the context of justification makes a hypothesis scientific, only it is relevant to the philosophical account of the rational warrant for scientific belief. Generally members of the consensus tended to relegate the context of discovery to the personal, and often idiosyncratic psychological factors which characterized particular historical human scientists, accounts of which may be psychologically or historically interesting, but which are irrelevant to the philosophical account of how a hypothesis becomes a part of science. The philosopher is interested in the "logic" by which a scientific belief comes to be warranted, not the psychological or historical process which led to its formulation in the first place. Thus the distinction between context of justification and context of discovery was also an essential element of the empiricist consensus in philosophy of science.

18. How can change in scientific belief be explained by the consensus?

One of the most obvious aspects of science is that what today's scientists believe differs markedly from the past, as the science of each period has differed from that of its predecessors. Scientific belief is not static, but continuously changing, perhaps often in extensive or revolutionary ways. Of course it can occur that laws which were formerly well confirmed by repeated tests, in the future get refuted by new tests, thus causing a change in the body of empirical evidence, which may have been once favorable but becomes unfavorable as human experience of the world grows. This is one kind of change. But often change was understood as a case of "reducing" more restrictive narrow hypotheses to broader more general hypotheses. According to the consensus it was ideally possible to reconstruct the history of any science showing that what were originally conceived as general laws turn out to be more restricted cases of yet more general laws. (This was especially held in the case where Galileo's mechanics was seen as a restricted case of Newtonian mechanics and Newtonian mechanics subsequently seen as a restricted case of Einstein's relativistic mechanics.) In this way, change in science could be explained in terms of progress moving towards the ever more general all-embracing fundamental laws of nature encompassing an ever greater range of phenomena. If laws are regarded as "true" this progress could be seen as progress towards the truth about the world we experience.

19. Are the fundamental laws true?

The word "truth" is generally taken to imply that what is true must be about something which exists or is "real." This is often called the "correspondence theory of truth", according to which the true statement is true because it represents things as they really are, the "facts" correspond to what the law says, whereas the false statement is false because it fails to do so. According to this notion of "truth" as referring to a representational relation between statement and reality, since all theoretical statements go beyond the purely observable and make statements about unobservable processes or properties or objects, perfected scientific theories are really true descriptions of this "unobservable" world which is often said to "lie behind the observable phenomena." This reality is often believed to stand in some causal relation to what is observed. In short it was held by the consensus that if one holds that such statements are true in this correspondence sense, then one must hold that such unobservable entities are the real causes of observable phenomena. This position is known as scientific realism. Many defenders of the consensus were scientific realists in this sense. However, one could accept all the elements of the consensus and argue that only observation statements are true or false; theoretical statements are neither true nor false, but are accepted because of their use as "instruments" enabling the scientist to infer from one set of observation statements, those stating the antecedent conditions, to another set of observation statements, those describing the observed phenomenon predicted or explained. This view, often called instrumentalism, is a specific form of "anti-realism," and was defended by some members of the consensus. Thus in one sense the consensus was neutral on realism versus anti-realism, but in another sense, namely that realism demands a commitment beyond what the consensus agreed upon, it is fair to regard the empiricist consensus as anti-realist, at least in spirit, though many of its defenders would have called themselves realists.

20. Does the consensus hold that it is describing actual science?

No; defenders of the consensus held that the philosopher of science was concerned with developing a "model" or account of an ideal perfected science. They fully well realized that actual real sciences always fell short, often severely, of this ideal picture of what a science should be. But this fact merely indicates that real science is incomplete and part of a human, historical process; thus it is heir to all the errors of which humans are capable.

21. What is the purpose of a philosophy of science which ignores real science and concentrates on a perhaps unattainable ideal?

The function of the philosopher of science is normative; the ideal picture of science painted by the philosopher serves as a standard by which one can assess the degree to which any body of beliefs approaches what it should be to gain rational acceptability, i.e. to be "scientific knowledge." At the same time this standard enables one to distinguish the bona fide true science from pseudo-science, and thus distinguish what the scientifically rational person ought to believe from what he ought to reject. The problem of drawing the boundary between science and pseudo-science was known as the "problem of demarcation" and was a central concern of many who held the empiricist consensus.