Because of their culture's ingrained emphasis on referring only to immediate, personal experiences, the tribesmen do not have words for any abstract concept, from colour to memory and even to numbers. There is no past tense, he says, because everything exists for them in the present. When it can no longer be perceived, it ceases, to all intents, to exist. "In many ways, the Pirahã are the ultimate empiricists," Professor Everett says. "They demand evidence for everything."