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.