Some things have to be believed to be seen.
The paradox here inverts our usual confidence in empirical certainty—it suggests that belief isn't the weak cousin of evidence but sometimes its necessary precursor. We often assume we must see something to believe it, yet L'Engle is pointing to those experiences where conviction actually enables perception: the parent who believes in their struggling child's potential and therefore notices capabilities others miss, or the scientist whose hypothesis shapes which phenomena she recognizes as significant among infinite possibilities. What makes this different from mere wishful thinking is that she's describing a real cognitive mechanism, not fantasy—our expectations genuinely alter what we're able to observe. That's why a therapist can hear hope in a client's voice while a skeptic hears only despair in the same words.