In Antoine de Saint-Exupéry’s The Little Prince, a fox tells the prince that “On ne voit bien qu’avec le cœur. L’essentiel est invisible pour les yeux.” (One sees clearly only with the heart. What is essential is invisible to the eye.)
I’ve seen the second half of the quote in a polymer text book. In chemistry, the essential part is invisible, even when scientists at first think that the visible properties of a substance tell the whole story. Sometimes what scientists think they know (from what is visible) is wrong and the invisible properties complete the story.