He tried to imagine what...
He tried to imagine what it would be like to have never known the light of day. He wasn't just thinking of being born underground in some artificial habitat. That, at least, would have artificial light. No, he was trying to comprehend what it would be like to have never known light of any sort, to have been born blind. Even as he tried, he realized the task was impossible. Having experienced light, he could never unexperience it. Even if he were to go blind at that very moment, he would still know the phenomenon described linguistically as light.