But, if all philosophical and scientific expressions are
hypothetical, and we have rejected all reasoning from necessary
causes, then from where can we derive a sense of reality? In
"Positivism and Realism," Moritz
Schlick provided an answer to the problem with what he called
"The Verification theory of meaning." He summarized it in these
1. The meaning of every proposition is completely contained
within its verification in the given.
2. This principle does not imply that only the given is
real. Such an assertion does not make sense.
3. Consistent empiricism does not deny the existence of the
external world; it merely points out the empirical meaning
of the existential proposition.
4. It does not assert that everything behaves as if there
were independent bodies. The subject-matter of physics is
not sensations but laws.
5. Therefore logical positivism and realism are not in
opposition. This makes anyone who acknowledges their
fundamental propositions an empirical realist
6. The opposition is between the empiricist and the
metaphysician whether realist or idealist makes no
7. The denial of the existence of a transcendental external
world would be just as much a metaphysical statement as its
Carnap attempted to operationalise Russell's pure logical
language so that it could be adapted to the verification theory
of meaning with the introduction of "protocol sentences,"
sometimes called observation sentences. These are simple
sentences derived from observations everyone can agree on. They
differ from Aristotle's first principles because those were
developed from inductive generalizations. These originate with
specific activities with specific repeatable consequences.
However, as logical positivists became more mature, it became
more and more obvious that they were not really less metaphysical
than Aristotle's first principles, at least not in the sense they
used the term metaphysics.
In "The Foundation of Knowledge",
Schlick attempted to put the problem of Carnap's protocol
sentences in a proper light. What was originally meant, he said,
by protocol statements, were statements that express the facts
with absolute simplicity in which every science consists and
which precede all knowledge and every judgment regarding the
world. Since there is no such thing as uncertain facts, these
Protocol statements then would become the absolute starting Place
for all knowledge. However, the only kinds of such statements
that can be made are statements to the effect that someone
observed something at some particular time. The result is that
while all of science eventually evolves down to such statements,
The truth of science and hence the source of knowledge does not.
The criterion of truth, he said was that all statements of
science must accord with each other.