The Quran

وَلَو نَشاءُ لَطَمَسنا عَلىٰ أَعيُنِهِم فَاستَبَقُوا الصِّراطَ فَأَنّىٰ يُبصِرونَ

Muhammad Asad

NOW HAD IT BEEN Our will [that men should not be able to discern between right and wrong], We could surely have deprived them of their sight,¹ so that they would stray forever from the [right] way: for how could they have had insight [into what is true]?²

Lit., "We could surely have effaced their eyes": a metaphor for "We could have created them morally blind" and, thus, devoid of all sense of moral responsibility - which, in its turn, would constitute a negation of all spiritual value in human life as such. (Cf. 2:20 - "if God so willed, He could indeed take away their hearing and their sight".)
In this instance - as, e.g., in 20:96 - the verb basura ("he became seeing" or "he saw") is obviously used in its tropical sense of "perceiving [somethingj mentally". According to Ibn 'Abbas, as quoted by Tabari, the phrase anna yubsirun signifies "how could they perceive the truth".