Statements can be justified only by other statements, and therefore testing comes to an end, not in the establishment of a correlation between propositional content and observable reality, as empiricism would hold, but by means of the conventional, inter-subjective acceptance of the truth of certain basic statements by the research community.
The acceptance of basic statements is compared by Popper to trial by jury: the verdict of the jury will be an agreement in accordance with the prevailing legal code and on the basis of the evidence presented, and is analogous to the acceptance of a basic statement by the research community:
By its decision, the jury accepts, by agreement, a statement about a factual occurrence—a basic statement, as it were. (2002: 92)
The jury’s verdict is conventional in arising out of a procedure governed by clear rules, and is an application of the legal system as a whole as it applies to the case in question. The verdict is accordingly represented as a true statement of fact, but, as miscarriages of justice demonstrate all too clearly,
the statement need not be true merely because the jury has accepted it. This … is acknowledged in the rule allowing a verdict to be quashed or revised. (2002: 92)
This is comparable, he argues, to the case of basic statements: their acceptance-as-true is also by agreement and, as such, it also constitutes an application of a theoretical system, and
it is only this application which makes any further applications of the theoretical system possible. (2002: 93)
However, the agreed acceptance of basic statements, like that of judicial verdicts, remain perennially susceptible to the requirement for further interrogation. Popper terms this “the relativity of basic statements” (2002: 86), which is reflective of the provisional nature of the entire corpus of scientific knowledge itself. Science does not, he maintains, rest upon any foundational bedrock. Rather, the theoretical systems of science are akin to buildings in swampy ground constructed with the support of piles:
The piles are driven down from above into the swamp, but not down to any natural or “given” base; and if we stop driving the piles deeper, it is not because we have reached firm ground. We simply stop when we are satisfied that the piles are firm enough to carry the structure, at least for the time being. (2002: 94)
https://plato.stanford.edu/entries/popper/