This was the topic of discussion between an historian, a mathematician and a mystic.
The historian said, "writing. The ability to put words on paper to be communicated to people who never even met the 'speaker', is the single greatest achievement of mankind."
The mathematician said, "no, numbers. The ability to express and develop truly abstract concepts, which in turn leads to Incredible real applications. Numbers are the single greatest invention of mankind."
The mystic said, "the Thermos flask."
"The Thermos flask?"
"The Thermos flask. It keeps hot drinks hot in the winter, and cold drinks cold in the summer. But think - that little flask - how does it know?"