Aaron Copland on Mozart

[W]e can pore over him, dissect him, marvel or carp at him. But in the end there remains something that will not be seized. That is why, each time a Mozart work begins . . . we composers listen with a certain awe and wonder, not unmixed with despair. The wonder we share with everyone; the despair comes from the realization that only this one man at this one moment in musical history could have created works that seem so effortless and so close to perfection.

Here is the Commentary article offering the quotation.