A great quote about postage stamps by Austrian artist Friedensreich Hundertwasser - whose own artwork has featured on many stamps for different postal authorities around the world:  

The stamp is an important object. Although very small in format, it carries a message. Stamps are a measure of the culture of a country. This tiny, rectangular piece of paper links the hearts of sender and receiver. It is a bridge between peoples and nations. The stamp knows no borders. It reaches us even in prisons, asylums, and hospitals, and wherever we may be on earth. Stamps should be ambassadors of art and life and not simply soulless proofs of postage paid. The stamp must experience its destiny. The stamp must once again fulfill its purpose, which means it must serve on letters. A true stamp must feel the tongue of the sender moistening its gum. A stamp must be stuck on a letter. A stamp must experience the dark depths of the post box. A stamp must suffer franking. A stamp must sense the hand of the postman handing the letter to the addressee. A stamp which is not mailed on a letter is no stamp. It has never lived, it is a sham. It is like a fish who has never swum, a bird who has never flown. A stamp must have lived as a stamp. The stamp is the only work of art that everyone can own, young and old, rich and poor, healthy and sick, educated and ignorant, free or robbed of freedom. This precious piece of art reaches everyone as a gift from afar. A stamp should be a testimony to culture, beauty, and the creative spirit of mankind.” – Hundertwasser, February 14, 1990