PST 2068
Spring 2003
Logical Positivism
Logical positivism is a philosophical movement of the early twentieth century. The movement had its roots in
the skeptical empiricism of David Hume and in the earlier positivism of August Comte, and it made use of the
new tools of symbolic logic developed by Frege and others.
One of the primary concerns of logical positivism is the PROBLEM OF DEMARCATION: What kinds of
claims are properly scientific, and which are not? The positivists imposed a stringent standard, such that any
statement not in the domain of the natural sciences is deemed to have no literal meaning – this includes all
statements of theology and metaphysics, as well as all value judgments.
This standard is the PRINCIPLE OF VERIFIABILITY: A statement is literally meaningful if and only if it is
VERIFIABLE.
DIRECTLY VERIFIABLE – “if it is an observation-statement, or is such that in conjunction with one
or more observation-statements it entails at least one observation statement which is not deducible from
these other premises alone.”1
INDIRECTLY VERIFIABLE – “first, that in conjunction with certain other premises it entails one or
more directly verifiable statements which are not deducible from these other premises alone; and,
secondly, that these other premises do not include any statement that is either analytic, or directly
verifiable, or capable of being independently established as indirectly verifiable.”2
It is a hallmark of logical positivism that the crucial issue is that of the relationships among statements, and that
those relationships must be expressible in terms of symbolic logic. (Also note that this formulation of the
principle of verifiability assumes that there is a separate class of statements that accurately record observations.)
Major contributors to logical positivism include Rudolph Carnap, Bertrand Russell, Ludwig Wittgenstein, and
A.J. Ayer.
Logical Empiricism
Some later philosophers in this vein began to loosen up the standard of meaningfulness even as they continued
to look for the logical underpinnings of scientific method. Their work is sometimes referred to as “logical
empiricism,” indicating a shift in emphasis away from the problem of demarcation.
Consider, for example, Carl Hempel’s treatment of what is sometime called the hypothetico-deductive method.3
The basic idea is that scientists can never start with raw observation; instead, they must already have a problem
in mind, as well as a proposed solution to that problem – a HYPOTHESIS.
A scientific hypothesis must have a TEST IMPLICATION – which is very much like what Ayer had in mind
with his principle of verifiability. In other words, the hypothesis must imply some observable state of affairs.
So:
If H is true, then so is I.
But (as the evidence shows) I is not true.
-----------------------------------------Therefore, H is not true.
1
A.J. Ayer, Language, Truth and Logic, 2nd edition (New York, Dover, 1952), p. 13
Ibid.
3
The following is distilled from Hempel, Carl G. Philosophy of Natural Science (Englewood Cliffs, NJ: Prentice-Hall, Inc.,
1966), pp. 7, 23
2
PST 2068
Spring 2003
This is a straightforward, logically valid argument of the form known as MODUS TOLLENS. But notice:
If H is true, then so is I.
(As the evidence shows) I is true.
---------------------------------Therefore, H is true.
This is an invalid argument of the form known as THE FALLACY OF AFFIRMING THE CONSEQUENT.
(Consider: If it is raining, then it is cloudy; it is cloudy, therefore it is raining.)
Things get more complicated, because the hypothesis never really stands alone; it is hedged in with assumptions
of AUXILIARY HYPOTHESES
If both H and A are true, then so is I.
But (as the evidence shows) I is not true.
-----------------------------------------Therefore, H and A are not both true.
So, even in the face of what seems to be a conclusive refutation, it is possible to save the hypothesis by
questioning or changing the auxiliary hypotheses.
On this account, scientific investigation can never demonstrably prove a hypothesis; the most it can do is
increase or decrease its probability.
Some Terminology4
A scientific LAW simply describes a regularity in our experience. The ideal gas law, for example, describes a
consistent relationship among the temperature, pressure, and volume of a gas, while Newton’s law of gravity
describes a consistent relationship between any two bodies with mass. In the physical sciences, laws are usually
expressed mathematically.
A scientific THEORY is a set of claims about entities, forces, and/or processes that cannot be seen that is
intended to explain regularities in the behavior of what can be seen. So, the kinetic theory of gases, which is
rooted in atomic theory, explains the ideal gas law, and the general theory of relativity explains the law of
gravity.
A HYPOTHESIS is a law or theory that is proposed to solve a problem, but which is not yet firmly established.
Note a consequence of these definitions: many creationists are fond of saying that evolution is “only a theory.”
Evolution is a theory that posits the existence of an unseen process (i.e., natural selection) that explains (among
other things) the diversity of life on earth. What creationists usually mean, however, is that evolution is an
unproven hypothesis. This is false, as the evolutionary theory as well established in biology as atomic theory is
in physics and chemistry.
4
See Hempel, 54, 70-71.