Albert Einstein: “Wer … nicht mehr staunen kann, der ist sozusagen tot und sein Auge erloschen.” [Wie ich die Welt sehe]
“He who … can no longer wonder, is as good as dead.” [The world as I see it, page 5]
and the full citation with a little more context:
The fairest thing we can experience is the mysterious. It is the fundamental emotion which stands at the cradle of true art and true science. He who knows it not or can no longer wonder, no longer feel amazement, is as good as dead, a snuffed-out candle.