Logical Empiricism
From optimism to surrender
The logical part: First order logic
• First order (quantified) logic combines the resources of
truth functional propositional connectives, a simple
subject-predicate analysis of sentences and (individual)
variables and quantifiers.
• The result is a formal language with quite a lot of
expressive power– enough for us to formalize
mathematical theories, for instance.
• With a language this clear and this powerful, the logical
empiricists hoped they could do what Hume and other
early empiricists could not: give a full account of all
(meaningful) ideas and concepts in terms of experience
and logic.
Experience
• Experience clearly plays a role in observation–
in ‘pure’ observation, many philosophers have
thought that we simply report what we
experience.
• Because such reports are often regarded as
incorrigible and infallible, this sort of pure
report of experience is often part of a
foundationalist picture of the epistemology of
‘facts about the world’.
Skepticism
• There are two characteristic forms of skepticism
that arise here.
• The first is about the links between an account of
what we experience and the external world:
How can the course of our private experience tell
us about the public, external world that (we
believe) causes most such experiences?
• The second is about induction and regularities:
How can reports about patterns of experience
we’ve had provide a basis for projecting those
patterns into future experiences?
Patterns of actual and possible
sensations
• One traditional response to the first sort of skepticism is to
re-define what an ‘external object’ is, in terms of ‘patterns
of actual and possible sensations’.
• This view is called phenomenalism.
• But it is widely accepted to be untenable.
• The problem is, we cannot describe these patterns
independently of our understanding of the (familiar) world
of physical objects.
• Kant’s account of our knowledge as combining fixed
conceptual structures (including space and time) with ‘raw
material’ from sensation solves this problem, if a bit
crudely. Other accounts focus on language and language
acquisition, which can be more flexible about concepts.
Science
• By this time, science is a standard example of
epistemic success.
• Einstein’s theory of relativity was a recent
example, undermining the ‘fixity’ of fundamental
concepts that Kant had assumed.
• The logical positivists proposed to apply their
logical tools together with empiricist ideas about
experience as the key to scientific knowledge, to
arrive at a new account of science that would fit
with Einstein’s work.
Philosophy of Language
• Analytic-synthetic distinction & knowledge of mathematics.
– Einstein’s use of non-Euclidean geometry in physics illustrates
the view that all mathematical knowledge is analytic (and all
synthetic knowledge is empirical). The difference between
applied and pure mathematics is central here.
• The verifiability theory of meaning.
– The meaning of a sentence is reduced to methods of deciding
whether the sentence is true or false. This turns out to be
untenable right away, since dispositions are meaningful, but we
often aren’t able to say whether an object has a certain
disposition or not. So weaker conditions were developed,
always maintaining a link between empirical testability and
meaningfulness.
Observational vs. Theoretical Language
• The positivists recognized the importance of
theoretical language, even though it seems
impossible to directly test theoretical claims
against experience.
• They proposed that theoretical language always
depended on a basic observational language for
its meaningfulness.
• The observational language would remain fixed
over time, ensuring that we always have an
agreed starting point we can apply to evaluating
and comparing different scientific theories.
Responding to Hume
• This requires an account of the logic of
induction– somehow, regularities in our
observations have to provide evidence that
certain observed patterns will persist.
• The positivists expected that logical work on
the relations between observations and the
theoretical claims those observations confirm
(or disconfirm) would reveal the structure of
inductive reasoning.
Limits of philosophy
• Context of justification: here logic is the key,
and we should be able to give an account of
our commitments and why we hold them.
• Context of discovery: here psychology is the
key; philosophy does not have to give an
account of how (rationally) we should go
about developing new hypotheses and
theories to test against our observations.
Positivists as bad guys?
• In some circles today ‘positivist’ is a term of
abuse– it implies a narrow, authoritarian
commitment to science, and a dismissal of
everything that science does not account for.
• Otto von Neurath’s ship metaphor is a good
starting point for appreciating how open-minded
and modest the positivists actually were.
• Better yet, this is an intellectual movement that,
in the end, realized that its goals could not be
achieved. How many such movements are clear
(and honest) enough to achieve such closure?
Holism
• W.V.O. Quine famously challenged the analyticsynthetic distinction in his classic article, “Two
dogmas of empiricism”.
• Quine’s response (in a move that some other
philosophers, including Sellars, also favoured)
was a kind of holism: our commitments face the
‘tribunal of experience’ together, rather than
individually.
• Further, the ‘analytic’ truths can also be changed,
if that’s the best way to accommodate
experience.
Late Logical Empiricism
• Theories are logically closed sets of sentences.
• They are connected to observation by ‘bridge rules’ that
link observations to the theoretical vocabulary.
• A successful theory will be confirmed by the combination
of observations and bridge rules.
• If there is trouble (a contradiction between the
observations made and the theory’s commitments) then
how to respond is decided holistically, by a kind of costbenefit calculus—any sentence in the theory can be given
up, if doing so accommodates experience better.
• Explanation is a matter of deriving some pattern in our
observations from other observations and theoretical
principles. (Deductive-nomological explanation)
The theoretician’s dilemma
• This paper by Hempel proposed (as Carnap also
maintained) that the contribution theoretical language
makes to our understanding of the world is exhausted
by the theory’s implications for patterns expressed in
the observation language.
• Thus, even though we have words like ‘electron’ in our
theory that seem to refer to strange, unobservable
objects which play a role in explaining what we
observe, the meaning of these words lies only in what
the theory (which uses them) implies about our
observations: science is about the ‘surface’ of things
only, not what lies beneath it. (See also Carnap, “The
methodological character of theoretical concepts”.)
What is hidden?
• Are electrons really hidden? Van Fraassen’s
example: high energy electrons can be
directly detected as flashes of ‘light’ produced
when they strike our retinas.
• Shapere: Observation in science and
(traditional) philosophy seem rather different:
Consider neutrinos and the interior of the sun.
Puzzled by induction
• Ampliative reasoning:
• Induction (from particular cases to
generalizations)
• Projection (from past cases to new cases)
• Explanatory inference (inference from data to
conclusions that explain the data– e.g. the
Alvarez hypothesis).
Hypothetical deductivism
• A theoretical claim is confirmed when
consequences of the claim are observed to be
true.
• In a simple version this is clearly wrong.
• But it fits so many familiar examples of good
science that it’s hard to give up altogether.s
A puzzle about some simple rules for
induction
• Nicod’s criterion: An instance of an A that is B
confirms all A’s are B’s.
• Equivalence rule: If some evidence confirms a
sentence P, it confirms any logically equivalent
sentence.
• ‘All Ravens are Black’ is equivalent to ‘All nonblack things are non-Ravens’.
• It follows that white sneakers confirm ‘All
Ravens are black’.
Goodman’s riddle
• Grue: Observed before or at t and green or
observed after t and blue.
• Bleen: Observed before or at t and blue or
observed after t and green.
• Let t be midnight, Decmber 31 2009.
• Then all observed emeralds are grue.
• Should we infer by simple induction that all
emeralds are grue?
Curve-fitting
• Given just some points on a graph indicating
observed relations between some quantities,
how do we ‘project’ to arrive at a general
pattern or rule relating them?
• These problems were never satisfactorily
solved, although there are many ideas about
how we might make some progress with
them.