I think the mathematician and philosopher thought that, in that way, his students might focus more on the voice, and his words and its meaning, rather than the visual of him speaking.
گمون کنم ریاضی‌دان و فیلسوف ما فکر می‌کرد، با این کار، شاگردانش احتمالا بیشتر روی صوت و کلمات و معانی اون‌ها تمرکز می‌کنن، نسبت به وقتی که چهره‌ش رو حین صحبت ببینن.