On the Connection between Copula “shi” and Focus Mark “shi”

In ancient Chinese, the pronoun shi (literally meaning "yes" in English) is not an ordinary demonstrative pronoun, but a specific pronoun. When shi is the backward anaphora to the front clause and acts as the subject of the following clause, its nature of reference becomes weaker and weaker, owing to the simplicity of the front clause, and then shi has a new usage of confirmation. Copula shi and focus mark shi are both extended from the confirmation usage. They were extended synchronously, and became mature in the medieval times.