The phrase “when the instruments are sound” highlights the role of the means through which information becomes accessible.
عِنْدَ سَلَامَةِ الْآلَاتِ
When the instruments are intact or sound.
A person without sight may still reason, though information acquired through vision is unavailable. Likewise, an impairment of hearing does not by itself eliminate rational capacity. Sensory access and the faculty of inference are related but distinct.
A defect in one channel may restrict particular kinds of input. It does not justify the sweeping claim that reason or every kind of knowledge has disappeared.