If a conclusion needs a proof, that proof relies on premises. We can ask for the premises' justification, and continue doing so. This is the problem of the regress of reasons.
Imagine asking “Why is this result true?” and receiving a premise. If every premise always required an entirely new premise, with no terminating source of knowledge, the original proof would never finish.
The response is that some basic cognitions must be available for any successful inference. Dependence on those cognitions does not automatically make inference defective; it indicates that both the starting points and the transitions between them require soundness.