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 seven steps.

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 difference.
7. The denial of the existence of a transcendental external world would be just as much a metaphysical statement as its affirmation.

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.