In here and there a bird, or butterfly,
Or flower, or wearing-stone, or open eye,
When heaven presents in sheets the solid hue?
Since earth is earth, perhaps, not heaven (as yet)--
Though some savants make earth include the sky;
And blue so far above us comes so high,
It only gives our wish for blue a whet.
Fragmentary blue, a poem by Robert Frost.
While for a physicist blue is the colour of light between violet and green on the visible spectrum, to a believer the vast blue sky is a glimpse of heaven. Blue doesn't exist in itself. Neither does heaven. Both are a result of perception and longing.
In eight pithy lines, using blue as a synecdoche, Robert Frost shows how human perception and longing are the basis of all meanings. We can't know the complete truth, but only fragments of it.