An argument can have a valid structure while relying on uncertain material. Formal validity alone does not turn probable premises into demonstrative knowledge.
Suppose someone reasons: “If the report is correct, the road is closed; the report is probably correct.” This does not yield “The road is certainly closed.” The leap from probability to certainty is unwarranted.
When premises are themselves certain and the inference is valid, demonstrative proof becomes possible. Rhetoric generally deals with premises below that threshold.