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 (or at least
appearing to explain) observable phenomena through appeal to general
statements about entities, states, or processes, which are postulated
(or "hypothesized") to exist but are not in fact observed,
and in many cases may 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.