Why the seals of the Indus Civilization are considered a form of writing?

The Indus Valley civilization was one of the greatest civilizations of ancient times. We get much of our knowledge about this civilization from over 2000 inscribed seals that have been discovered in good condition. The Indus people did not engrave inscriptions on stones or place papyrus scrolls in the tombs of their dead. All we know of their writing is derived from the simple inscriptions on their seals. The inscriptions are in the form of some 400 signs. The signs are the written language of these people.These seals tell us a lot about the way the people lived, the gods and goddesses they worshipped, the animals they reared and, other interesting bits of information about their culture