How does Hypothetico-Deductivism attempt to Justify Laws and Theories?

Many defenders of the empiricist consensus  rejected inductivism  in favor of a hypothetico-deductive or "H-D" model of justification of laws and theories.  According to this analysis, the process of justification of theories starts with a conjecture about the world, the hypothesis, and then science proceeds to deduce observable consequences of the hypothesis given a certain precise specification of the "initial conditions."  Experiments are then contrived to bring about such conditions and precise observations are performed to determine if the predictions from the hypothesis are in fact
observed to occur. If so, we speak of the hypothesis as "confirmed," if not, it is "refuted."

     Hypothetico-deductivism has two epistemological characteristics in common with inductivism:
both are empiricistic and both are foundationalistic, both of which are positions essential to the
empiricist consensus.  H-Dism is empiricistic because, even though scientists begin with a deduction
from theoretical hypotheses which are not claimed to have been induced from empirical observations,
the authority which a hypothesis commands once it becomes "accepted" as a "law," it commands only
because of its justification by empirical confirmation.  Thus though explanation and prediction flow
"downwards" deductively from hypothesis [at the "top" of the pyramid] to observation statements, "authority"
("justification") flows "upwards" from empirical observations [at the "foundation" of the pyramid] to the
hypotheses they confirm.  Because authority thus flows upwards from observation to law, an H-D advocate
is correctly considered a kind of empiricist, though not of the classical inductivist sort.

     Furthermore, like inductivism, H-Dism holds that general, theoretical knowledge is knowledge
because it is erected upon a "foundation," the "given" truth of which is taken as more or less
unproblematically established by observation.  This foundation is thought to consist of a finite array of
"observation statements" each containing only observational terms and each "verified" directly by
observation.  Precisely what it is which is "directly observed" and thus serves to "verify" an observation
statement is not determined by H-Dism, and those who defend this model of justification can and do
differ dramatically about what they hold such "verifiers" to be.  Nevertheless, whatever these verifiers,
the basic "observation statements," might be, they serve as the "foundation" on which the structure of
empirical knowledge is erected.

     For both these reasons, H-Dism is just as much committed as is inductivism to the crucial
observational vs. theoretical distinction between different kinds of statements.  However H-Dism avoids
both the problems which haunted inductivism:
a) There is no need to develop an inductivist logic from which one infers to theories from initial observations; so the "problem of induction" does not arise.

b) Theory-ladeness of observation does not matter. This is so because according to H-Dism the theory comes first before the observations. However, it starts out as nothing more than a hypothetical conjecture which awaits confirmation by observation. It comes to be an accepted theory (and hence enter the ranks of "knowledge") only after some period of more or less extensive confirmation.  Since the scientific method starts with a hypothesis, that hypothesis directs the scientist to which data to gather and provides a language in which to express it.

     The H-D switch from proof to confirmation entails an abandonment of the historical goal of
certainty and a pragmatic orientation which inquires into the degree of rational justification of a
scientific theory.  Thus H-D advocates are "falibilists" who admit that any theory, no matter how well
confirmed is potentially open to falsification in the light of new tests, and hence was not knowledge.
However, as they well knew, by and large once a hypothesis has passed a period of being tested and
comes to be an accepted theory, the testing work of science leaves that hypothesis and advances to a
concern with new ones.  The general picture of scientific knowledge which one got from this accorded
well with the pyramidal construction of explanation from an apex of a few high level theoretical
principles to manifold observation statements at the base.  Growth in scientific knowledge was seen as
cumulative extending the base of the pyramid with more and more empirical evidence, and its height
by more and more all embracing general theoretical principles.
     On the H-D view, the scientist no longer appears to be a "prover of theories from the undirected
accumulation of data" but now appears as a "tester of hypotheses by controlled observation".  Thus
science does not start with accumulating data randomly; it starts with a hypothesis which directs what
data to acquire and provides a language or "conceptual framework" to express it in.
Though the DN model of explanation does not require it, most advocates of hypothetico-deductive model of justification assumed the DN model. The validity of the argument which explains a phenomenon only assures us that the explanandum follows from the explanans, but is the explanans "true"? If scientific explanations are to be considered as giving us knowledge, don't we have to know their premises are true?  Would a successful prediction from a false hypothesis count as scientific knowledge?
    Consider the three kinds of statements in the D-N model of a scientific explanation: the initial conditions statements can be (presumably) "verified" by direct observation.  So let us take their truth as unproblematic (at least most H-D advocates tended to do this).  Thus our model of explanation simplifies to Hypothesis => Explanandum.  Let us also assume the statements describing the explanandum can be verified directly by observation.
     What then can we know about the truth of the hypothesis?  It can't be verified directly by observation because it includes what aspire to be candidates for general laws ("law-like" statements). By definition a law is a universal statement, and if it is not a purely empirical law, it will have in it theoretical terms which do not refer to anything directly observable.  Can we infer from the truth of the explanandum, i.e., from the fact that we observe the phenomenon predicted by deduction from the hypothesis in question, the truth of the hypothesis?  No, to do so is a logical fallacy known as "affirming the consequent."  From the falsity of the explanandum (i.e. observation reveals that the prediction was not true), we can validly infer the falsity of the hypothesis, but from the truth of the explanandum we cannot infer the truth of the hypothesis.

In other words, even though we may validly deduce a statement describing observable phenomena from a hypothesis, i.e., predict a specific phenomenon, and observation reveals that the phenomenon occurs as predicted, we still cannot conclude from this fact that the hypothesis is necessarily true. Other hypotheses, incompatible with the original one, may also equally well predict the same phenomenon exactly as it is observed to occur. In fact it is possible to prove logically that there are an infinite number of logically possible hypotheses from which that very same observed phenomenon could have been equally validly deduced. This proof is known as "Craig's Theorem."

We express this perhaps surprising conclusion by saying that the empirical evidence "underdetermines" theory choice (note that this is not "undetermines" but "underdetermines").  This conclusion is known as the "Thesis of the underdetermination of theory by empirical ecidence."

     Thus though observation can refute a hypothesis, it can never verify (i.e., prove it to be necessarily true) any
hypothesis.  Consequently, like modern day inductivists, the H-D theorist also must reject the traditional
goal of certainty as the criterion of knowledge, and with this move the scientific method no longer
becomes a tool for "verification" or "proof." Instead H-D theorists must now speak in terms of
"confirmation" (or in the case of Popper "corroboration," defined as the absence of refutation).

    The question that philosophers of science now address changes from "How do we prove the truth of this theory with the data we've got?" to "On the basis of what empirical evidence do we conclude that it is most rational to accept some one particular hypothesis out of all those hypotheses which have been proposed, tested, and not refuted by H-D methodology?"

     Notice that insofar as the "testing of hypotheses" now becomes the central epistemic task of
science, the distinction between observational and theoretical terms remains crucial.  Since it is the
"observations" which pass judgment on the "hypothesis" the degree of credibility, or "acceptance," which
any hypothesis achieves, it will achieve only if the observational evidence is "independent" of the
hypothesis on which it is used to pass judgment. To the extent that we can show that the observational
evidence is "laden" by theoretical beliefs in a way which cannot be eliminated, we undermine the
"objectivity" of the test and thus the justification of the hypothesis.  Had we accepted a different theory
to begin with, we would have gathered different observational evidence (which that that theory led us
to obtain), which may well have led to the confirmation of that theory.

     Another crucial move in embracing H-Dism involves the question of where hypotheses come
from in the first place.  In order to avoid the problems of inductivism, H-Dism is anxious to stay away
from questions like this, because it does not want to make the hypothesis appear to be "induced" from
prior data.  H-Dism effectively moves to "table the question" as far as philosophy of science is
concerned.  Thus H-D theorists argued that it doesn't matter where the hypothesis comes from because
what makes it scientific is not its origin, where it comes from, but the fact that it has been subjected
to rigorous repeated testing - and passed (or wasn't refuted)- which turns a mere hypothesis into a
scientific theory fit for the title of "knowledge."  Therefore, the philosophical justification of scientific
knowledge does not need to ask where the hypothesis "comes from." Of course scholars interested in
history or the psychology of science might investigate such matters, but whatever they might find there
has no bearing on the philosopher's goal of showing that science provides knowledge.

     Another way to put this is as follows: Inductivism holds that theory, to speak loosely, comes
from empirical data, but this gives rise to the problem that if there is at first no theory, one has no idea
what data to gather.  So H-Dism holds that theory, in the form of a conjectured hypothesis, precedes,
and therefore directs, the accumulation of data, which in turn then confers confirmation on the
hypothesis, turning it (if it is succesful) from a speculative conjecture into an accepted "law" or "theory."

However, if one asks "Where does the hypothesis come from?" the H-Dist can answer that such a question is for the
"context of discovery" and is a totally separate issue from the question of the "logic of confirmation"
with which the "context of justification" is concerned.  As philosophers concerned with answering the
question of authority we need only address issues in the context of justification, for only justification
makes conjectures into scientific knowledge.  The origin of those conjectures, the concern of the
"context of discovery" is no doubt an intriguing area for psychological, historical, and/or sociological
analysis, but for defenders of the consensus, it is irrelevant to our philosophical concerns.
(Unfortunately the tendency of just this sort of  allegedly irrelevant hisotrical and psychological research has been to undermine the whole empiricist presuppumption that there is any "logic" of justification at all.)

     Consequently, in embracing H-Dism, the empiricist consensus erected a very powerful distinction
between what was called the "context of justification" and the "context of discovery."  The "context of
justification" was concerned exclusively with the testing of hypotheses (taken merely as "given") by the methodology
of deducing observational tests and their confirmation (or refutation) by observation.  This was regarded as a properly
philosophical (epistemological) matter.  The "context of discovery" refers to questions concerning the
origin of hypotheses by the efforts of "creative imagination" and the psychology of scientific genius,
matters which were regarded as appropriate subjects only for historical and psychological research.

     Under this distinction, history of science has little or nothing to do with philosophy of science.
The philosopher may, of course, point to various episodes in the history of science to show that the
methodology he has prescribed is exemplified by great theoretical achievements.  But if historical records
and methodological prescriptions do not agree, the philosopher can always take refuge in the claim that
his business is normative; he prescribes the methodology that scientists ought to follow.  Of course
real historical scientists are only human and therefore do not always do what they ought to do. The
philosopher is concerned with erecting a model of an "ideal" science, which real science may more or
less perfectly embody, not with painting a picture of real, historical, imperfect science..

Problems with Hypothetico-Deductivism:

    H-Dism can be, and was, attacked on several different fronts.  First, there were problems
"internal" to the empiricist consensus that raised questions about the degree of acceptance that should
be accorded any general claim on the basis of any body of observational evidence.  Carl Hempel and
Nelson Goodman formulated a variety of paradoxes in the H-Dist attempt to avoid the pitfalls of inductivism.
 See "The Paradoxes of the Gruesome Ravens"  There are still, however, those philosophers who hold out
hope for theories of confirmation and a good bit of research still pursues this goal.

     Secondly, H-Dism, and all empiricistic foundationalism, can be attacked externally at its
foundation with respect to its rather naive acceptance of the incorrigibility of allegedly given
observational data.  The recognition that observation is essentially "theory-laden" did much to undermine
H-Dism's claim that the acceptability of a hypothesis is solely a function of the empirical evidence.
See the Argument for Theory-Ladeness of observation

     Finally, H-Dism's absolute separation between the context of discovery and the context of
justification can (and, according to many, did) lead to a reconstruction of science that bears little
resemblance to the historical record left by real science and allows no philosophical understanding of
the process by which hypotheses are generated prior to their being tested.