We can start with the distinction between observational and theoretical vocabularies.  Many empiricists were wedded to the myth of the given, assuming that there is a privileged observation vocabulary.  The meanings of observation terms were determined by their relation to what is given and were thus unrevisable or incorrigible.  This vocabulary grounds the meaning of all empirical language.  Two kinds of generalizations were recognized: (1) empirical generalizations formulated in the observation language and justified by straightforward inductive methods, and (2) higher-level generalizations formulated in a more abstract, theoretical idiom including novel theoretical posits.  This theoretical language consists of terms invented in order to better organize, generalize, and make connections among the assertions and generalizations made in the observation language.  The ideal would be a formal axiomatic structure that would allow a deduction of the empirical generalizations which, combined with statements of initial conditions, permit a deduction of particular empirical facts (thus explaining their truth). 