Hempel, The DeductiveNomological Model of Science

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Hempel, The Deductive-Nomological Model of
Science

• Deductive-Nomological Model of Explanation
  explanations are sound deductive arguments.
• D-N Model of Confirmation the explanation of
  some fact is also a prediction of that fact.

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D-N Model of Explanation

• explanandum sentence sentence describing the
  event or phenomenon to be explained
• explanans sentences contain the explanatory
  information
• In a good explanation, the explanans deductively
  implies the explanandum.

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Explanans sentences

• The explanans consists of two kinds of sentences
• 1. General laws that express uniform empirical
  connections.
• 2. Descriptions of facts.

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Schematic for D-N Explanation

• L1,...,Ln (General laws)
• C1,...,Cm (Descriptions of facts)
• E (Explanandum event)

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Summary on Explanation

• Hempels examples are designed to show that the
  D-N model accounts for how both particular facts
  and uniformities in nature can be explained by
  science. Namely, by using a deductive argument
  in which the thing to be explained, the
  explanandum, is the conclusion.

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D-N Model of Confirmation

• The explanation of some fact is also a prediction
  of that fact.
• When a prediction is born out by observation,
  that is some confirming evidence for the theory.

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Problem for D-N Confirmation

• The evidence for a theory is minimal and mainly
  negative, based on the following modus tollens
  argument
• P1. If (L1,...,Ln and C1,...,Cm) then E.
• P2. Not E.
• C. Therefore, not (L1,...,Ln and C1,...,Cm)

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Modus Tollens

• Modus Tollens is a valid deductive argument form
• If p then q.
• Not q.
• Therefore not p.

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Example of Modus Tollens

• If the body theory of personal identity is
  correct, then one is not the same person whenever
  one loses body parts. But losing ones hand does
  not make one a different person. Therefore the
  body theory of personal identity is not correct.

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Structure and Purpose of Scientific Theories

• explain a set of uniformities as a manifestation
  of some underlying processes and entities.
• specify clearly and precisely the basic entities
  and processes that occur in nature
• examples Newtonian physics kinetic theory of
  gases

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Theories, continued

• This clarity and precision allows deduction of
  regularities and hence tests of the theory.
• Thus the clarity and precision of scientific
  theories sets science apart from unscientific
  theories.

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Derivation of Predictions

• Theory must specify both internal principles
  (which characterize basic entities, processes,
  and laws of theory) and bridge principles.
• The bridge principles tie the theoretical terms
  to the pre-theoretical or everyday terms.
• Prediction is derived by deduction from theory
  and initial conditions.

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Virtues of Theories

• Theories broaden and deepen understanding of
  nature in three ways
• 1. offer a systematic, unified account of diverse
  phenomena
• 2. show how previous theories hold approximately
• 3. predict and explain new(ly discovered)
  phenomena

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Problems with Hempels Picture of Science

• 1. Most predictions/explanations in science are
  not deductive
• This led Hempel to introduce the
  Inductive-Statistical model
• L1,...,Ln
• C1,...,Cm
• implies with high probablility
• E

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Problems, continued

• 2. However, as Popper argues, induction is a
  flimsy method of confirmation of theories or laws
  
• 3. Laws are astonishingly difficult to
  characterize