ἁπλοῦς  [2 verses] (adj sg masc nom) "Single" is from haplous, which, as an adjective, it means "twofold", "single", "simple", "plain", "straightforward", "simple", "open", "frank", "simple-minded", "unalloyed[metals]," and "pure[metal]. - - - The Greek word translated as "single" primarily means "single," but it also means "open," which applies to eyes, but it means "open" more in the sense of being "frank" from its meaning as "straightforward." The sense here of that could be "seeing straight."  The "single" could also mean "focused" when applied to eyes.