The Eyes Cannot Look Two Ways and See Clearly

‘The eyes cannot look in two directions and see clearly; the ears cannot listen to two things and hear keenly.’ The sentence comes from Xunzi’s Encouraging Learning. It follows examples of taking diverging roads and serving two rulers, then leads into contrasts between single-minded concentration and having many skills without mastering them. It does not claim that people can never handle two things at once. Its target is divided purpose in learning. A restrained modern reading is not to reject every switch of attention, but to keep one aim during the moments that require judgement and understanding, then turn deliberately to the next task.

https://zh.wikisource.org/wiki/荀子/勸學篇