A.T. Robertson Commentary John 11:10

"But if a man walk in the night, he stumbleth, because the light is not in him." — John 11:10 (ASV)

But if a man walk in the night (εαν δε τις περιπατη εν τη νυκτ). Third condition again. It is spiritual darkness that Jesus here pictures, but the result is the same. See the same figure in 12:35 (1 John 2:11). The ancients had poor illumination at night as indeed we did before Edison gave us electric lights. Pedestrians actually used to have little lamps fastened on the feet to light the path.

In him (εν αυτω). Spiritual darkness, the worst of all (cf. Matthew 6:23; John 8:12). Man has the capacity for light, but is not the source of light. "By the application of this principle Christianity is distinguished from Neo-Platonism" (Westcott).