A rational conclusion requires sound premises and an appropriate inferential link between them. Invoking “reason” does not turn an unsupported guess into knowledge.
For example, “All humans are mortal; Zayd is human; therefore Zayd is mortal” has a valid structure, given its premises. “This object shines; therefore it must be gold” does not: many things shine.
Sound reflection (naẓar ṣaḥīḥ) means disciplined reasoning under appropriate conditions. The epistemic defense concerns this kind of inference, not every thought that happens to occur.