Laozi is the only figure in this collection whose existence is doubted by the tradition's own earliest biographer.
Sima Qian, writing the Records of the Grand Historian around 100 BCE, gives the account everyone repeats: a keeper of the archives at the Zhou court, named Li Er, who met Confucius and rebuked him, who grew weary of the decline of the state, rode west, and was stopped at a mountain pass by a gatekeeper who asked him to write down his teaching before leaving. He produced five thousand characters in two parts and went away, and nobody knows where he died.
Sima Qian then does something remarkable. He offers two further candidates, Lao Laizi, a contemporary of Confucius, and a court historian named Dan who lived over a century later, says that the world cannot tell which of them Laozi was, and concludes that Laozi was a hidden gentleman. The founding biography is explicitly uncertain about who its subject was.
The name is not a name. Laozi means old master or old child, a title rather than a personal designation.
Modern scholarship is largely comfortable treating the text as a composite work, assembled from a body of aphoristic material over a period, rather than as the product of a single afternoon at a mountain pass. The manuscript evidence supports layered growth: the Mawangdui silk manuscripts of about 200 BCE have the two parts in the reverse of the received order, and the Guodian bamboo slips from a tomb closed around 300 BCE contain roughly a third of the received text in a different arrangement.