The Theory Ladeness of Observation

Look at a clock and note the time.  Now you have a belief about the time of day.  Your belief is justified by observation but not entirely by observation.  Lots of other beliefs are also involved in supporting you belief about the time.  These include your belief about the clocks accuracy, your beliefs about the reliability of your own senses, your (semantic?) beliefs about what times are represented by what clock states and perhaps others yet.  An internalist theory of justification might require that adequate justification for your belief about the time includes belief contents which taken together would deductively entail your belief about the time.

The Positivist's view of science invoked a sharp distinction between observation statements and theoretical statements.  Observation statements contained no theoretical terms.  Since observation statements contained only words defined in terms of observation and logical vocabulary, observation statements were claims whose truth or falsity could be determined by observation alone, without appeal to theory.  That is, observation claims are justified solely in terms of observation and not in terms of statements of scientific theory. 

Norwood Russell Hanson was the first to argue that in fact observation claims are not justified by observation alone, but justified also by bodies of background theory.