the eye is blind if the mind is absent
Proverb meaning that without mental attention, one cannot truly see or understand what is before one's eyes. Classical expression, rarely used in everyday conversation.
The saying 'the eye is blind if the mind is absent' teaches the importance of concentration.
A modern colloquial expression with a similar meaning: 'looking but not really seeing.'
From the Confucian classic 『大学』: 心不在焉、視而不見, “when the mind is not present, one looks but does not see.”