Theories in Science


theory (scientific)—a set of principles that seeks to explain uniformities, expressed in the

     form of empirical laws, as manifestations of underlying entities and



          example: the kinetic theory of gases—

                   explains empirical uniformities among pressure,

                   temperature, volume, etc. by reference to the behavior

                   of atoms and molecules


2 kinds of theoretical principles:

  1. internal principles—describe the “underlying entities and processes” (e.g., atoms, molecules, fission)
  2. bridge principles—relate the underlying entities and processes (described by internal principles) to empirical phenomena with which we are already acquainted (e.g., principles relating charged particles (electrons) to voltage and current)


“empirical phenomena”—

        The empirical phenomena with which we are already acquainted need not be directly observable.

        Our acquaintance with the empirical phenomena may occur because of previously established theories (e.g., gravity).


According to Hempel, without bridge principles

        Theories would have no explanatory power.

        Theories would not be testable.


Theoretical and Pretheoretical Terms


theoretical terms—terms that refer to the underlying entities and processes of a theory (e.g.,

    “neutrino,” “natural selection”)

pretheoretical terms—terms that refer to the empirical phenomena with which we are

already familiar (e.g., “volume,” “temperature,” “mass”)


Characteristics of good scientific theories (according to Hempel):

1.      are testable in principle

2.      have explanatory import

3.      provide unified accounts of diverse phenomena

4.      show the limitations of the empirical laws that they are meant to explain

5.      predict and explain previously unknown phenomena


Important Question:
Are theoretical entities and processes (e.g., viruses, DNA recombination) real?


Arguments for “No” answer—


1.      Theoretical terms cannot be defined in terms that are already available and understood.


Hempel’s reply—

        A term can be meaningful even if it cannot be defined.

        Other criteria for the use of theoretical terms are available (e.g., bridge principles)


2.      (1) When 2 competing theories equally account for the same data, if the theoretical

  entities of one theory are real, then the same should be said of the different

  theoretical entities of the other theory.

      (2) However, this is implausible.

     (3) Therefore, theoretical entities are mere fictions.


Hempel’s reply—

       The fact that we cannot establish a theory with certainty does not mean that the

 theory is not true or that the entities to which it refers are not real.


3.      Theories that go beyond the facts of our experience are “formal devices” and should not be considered as representing reality.


 Hempel’s reply—

a.       Since scientific theories are tested and confirmed in basically the same way as

 nontheoretical scientific laws, it seems arbitrary to regard the entities of laws as

 real but those of theories as fictions.

b.      Any line drawn between “observables” and “unobservables” is arbitrary.