 All these practices and methods were current during the Later Han period and the Six Dynasties (1st-6th centuries CE).The second kind of criticism is addressed to alchemical practices that are not based on the principle of "being of the same kind" (or "category," "tonglei" 同類).Only Lead and Mercury, according to the "Cantong qi", are of the "same kind" as Qian and Kun, and can represent and enable their conjunction.