Some Ming Dynasty Taoist sects believed that one way for men to achieve longevity or 'towards immortality' is by having intercourse with virgins, particularly young virgins. Taoist sexual books, such as the Hsuan wei Hshin ("Mental Images of the Mysteries and Subtleties of Sexual Techniques") and San Feng Tan Cheueh ("Zhang Sanfeng's Instructions in the Physiological Alchemy"), written, respectively, by Zhao Liangpi and Zhang Sanfeng (not to be confused with semi-mythical Zhang Sanfeng who lived in an earlier period), call the woman sexual partner ding (鼎) and recommend sex with premenarche virgins. Zhao Liangpi concludes that the ideal ding is a premenarche virgin just under 14 years of age and women older than 18 should be avoided.[18] Zhang Sanfeng went further and divided ding into three ranks: the lowest rank, 21- to 25-year-old women; the middle rank, 16- to 20-year-old menstruating virgins; the highest rank, 14-year-old premenarche virgins.[19]