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The Observable / Unobservable Distinction. Anti-realism depends crucially on this distinction – the epistemological thesis. A somewhat confusing distinction: “Observable / theoretical” distinction Can we divide our language into a theoretical and non-theoretical part?

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the observable unobservable distinction
The Observable / Unobservable Distinction
  • Anti-realism depends crucially on this distinction – the epistemological thesis.
  • A somewhat confusing distinction:
    • “Observable / theoretical” distinction
    • Can we divide our language into a theoretical and non-theoretical part?
    • Can we classify objects and events into observable and unobservable ones?

Some relevant questions:

    • Is the line drawing the observable/unobservable distinction arbitrary?
    • Is this distinction epistemologically significant?
    • Does this distinction really exist?
    • Is everything “observable in principle”?

It seems that cloud chambers only allow us to “detect” electrons, but not “observe” them.

  • E.g. a jet aircraft can only be “detected” by the trail left behind.

But how to distinguish between observation and detection? Would it be arbitrary?

  • Consider these sequence of events: looking at something
    • with the naked eye,
    • through a window,
    • through a pair of strong glasses,
    • through binoculars,
    • through an ordinary microscope,
    • through an electron microscope, and so on.


Not observing

  • Some argue that these events lie on a “smooth continuum”:
  • How to decide which count as observing and which not?

E.g. can a biologist ‘observe’ microorganisms with a high-powered microscope, or can she only ‘detect’ their presence?

  • Detection relies heavily on theory.
  • Interpreting this image also relies on theory.
  • Theory-ladenness of detection / observation


Not bald





  • So how much theory-ladenness is needed to draw the line? Arbitrary?
  • Yet some retort that this argument only shows that “observable” is a vague concept.




  • The same applies to “observable”:
  • Vagueness seems only to set an upper limit on the precision with which antirealists can formulate their position.
    • What do you think?
the underdetermination argument

E.g. Boyle’s Law

Directly tested by observing

the readings on the apparatus.

Molecules postulated

are unobservable.

The Underdetermination Argument
  • Why is knowledge of unobservables impossible?
  • Consider again the kinetic theory of gases:
  • Observational data constitute the ultimate evidence for claims about unobservable entities.



Any relevant observational data

  • Anti-realists argue that:
    • Observational data “underdetermine” scientific theories:
    • If this thesis is true, agnosticism about the unobservable world is vindicated.

Underdetermination & the Duhem-Quine thesis:

    • Pierre Duhem (1861-1916)
      • French philosopher, historian
    • W. V. O. Quine (1908-2000)
      • American philosopher, logician
    • Recall some previous points:

Negative evidence does not prove conclusively that a theory is incorrect.

(Theory T • Auxiliary Hypothesis H)  Implication I

 I ___.

 (T • H)


Example 2: The discovery of Neptune

    • Newton’s theory of gravitation wrongly predicted the orbit of Uranus.
    • Some scientists tried to rescue Newton’s theory by postulating the existence of an unknown planet.
    • Neptune was finally discovered at almost the exact place and time as predicted.

T + H

T’ + H’

any relevant

observational data

  • The Duhem-Quine thesis:
    • Any theoretical claim T can consistently be retained in the face of contrary evidence by making adjustments elsewhere in our web of beliefs.

(T • H)  I (T • H’) I

  • Underdetermination:

Both T and T’ can

always be retained!


Besides observational data, recall the criteria of adequacy (pragmatic virtues) for choosing theories:

    • Simplicity, conservatism, fruitfulness, scope.
  • But are such criteria relevant to the truth of theories, or at least to their probable truth?
    • What do you think?
  • Reference:
    • “Simplicity” from the Stanford Encyclopedia of Philosophy



The camera that took the

pictures was faulty

Caused by UFO

The large crater

appearing in 1987

The large crater

appearing in 1987

A meteorite struck

the moon in 1987

  • A common hypothesis:
  • Many alternative hypotheses are possible, however:

The Duhem-Quine thesis guarantees that all these hypotheses can be sustained come what may.

  • Hence underdetermination again!
  • Only knowledge of actually observed things is possible?!
  • But what about knowledge of
    • dinosaurs,
    • continental drift,
    • formation of the Earth,
    • and many other things in science?

Seems to be a reductio ad absurdum.

    • Knowledge of the unobservable world seems possible after all.
  • But a version of the problem of induction still persists:
    • The problem of induction:
      • How to justify inductive inferences - on which scientific knowledge heavily depend?
      • Uniformity of Nature Assumption (UNA)
      • No consensus.
      • Scientific knowledge is ultimately founded on faith towards UNA?





Theory about unobservable

or unobserved entities


observational data

  • Underdetermination:
  • Hence inductive inference from data to theory:

Final remarks:

    • The underdetermination argument does raise a real difficult problem.
    • But it seems that there is no specialdifficulty about unobservable entities.
    • Problem of inductive knowledge for all sorts of objects.