Whorf's study of Hopi time has been the most widely discussed and criticized example of linguistic relativity. In his analysis he argues that there is a relation between how the Hopi people conceptualize time, how they speak of temporal relations, and the grammar of the Hopi language. Whorf's most elaborate argument for the existence of linguistic relativity was based on what he saw as a fundamental difference in the understanding of time as a conceptual category among the Hopi. He argued that the Hopi language, in contrast to English and other SAE languages, does not treat the flow of time as a sequence of distinct countable instances, like "three days" or "five years", but rather as a single process. Because of this difference, the language lacks nouns that refer to units of time. He proposed that the Hopi view of time was fundamental in all aspects of their culture and furthermore explained certain patterns of behavior. In his 1939 memorial essay to Sapir he wrote that “... the Hopi language is seen to contain no words, grammatical forms, construction or expressions that refer directly to what we call 'time', or to past, present, or future...”