What makes a doubt that stays with you?

I’ve been thinking about the kind of doubt that doesn’t go away, not because it’s loud, but because it’s quiet—like the hum in the walls when the power’s off. It’s not about faith or disbelief, exactly. More like wondering if I’ve ever truly seen someone, or if I just built them out of half-remembered gestures and the way light fell at 5:17 p.m. on a Tuesday. What’s your version of that? The one that doesn’t need an answer.