Sellars thinks that this instrumentalist picture gets almost everything wrong.  In his view the observation vocabulary/theoretical vocabulary distinction is merely methodological and is, moreover, highly malleable; it therefore possesses no particular ontological force.  There is no given, so it can play no semantic role.  Meanings are functional roles in language usage, and nothing in principle prevents a term that might originally have arisen as part of a theory from acquiring a role in observation reports.  The well-trained physicist “just sees” an alpha-particle track in a cloud chamber as directly and non-inferentially as the well-trained child just sees a dog.  Furthermore, what is observable depends on the techniques and instruments employed, and these are often loaded with theoretical baggage.  “Pure” observation uncontaminated by theory is outside our reach.  A good theory, according to Sellars, does not just explain the observation-level empirical generalizations, it also explains why those empirical generalizations are as good (and as wrong) as they are.  The Boyle-Charles ideal gas law works well in most conditions, but does not predict the behavior of gasses under extreme temperatures or pressures.  The van der Waals equation, explicitly formulated with the theory of statistical thermodynamics in mind, allows us to see why the Boyle-Charles law is as good as it is and why it breaks down where it does.  Sellars therefore rejects the “layer-cake” view that there are three distinct levels of assertion in the empirical sciences: observation level claims of particular fact, empirical generalizations using the observation vocabulary, and, finally, claims using a theoretical vocabulary which function to systematize the empirical generalizations.  In the “layer-cake” view, theories are ultimately dispensable in favor of observation-level empirical generalizations; in Sellars’s view, it is the observation-level empirical generalizations that are ultimately dispensable in favor of the better explanations made available by theory.  Thus, to have good reason to accept a theory is to have good reason to believe in the existence of those entities it postulates.