薛道光（1078-1191），字式，号道源，出身陕西鸡足山人氏，是南宗三祖的重要人物。南五祖宝诰中称其为道光紫贤真人。薛道光本是一个修行的僧人，他法名紫贤，在俗间人们尊称他为毗陵禅师。在他的漫游生涯中，他曾云游长安，并在开福寺留下了深刻的印记。他不仅参拜了许多高僧，如环，还有幸与修岩长老共度时光。在一次偶然的机会里，当他目睹桔槔轧响时，不禁心灵大开，顿悟无上圆明真实法要。
自此之后，他机锋敏捷、宗说兼通，就如同六祖慧能悟性后所追求的黄梅传法一样。他相信金丹修命之本必有一定的秘传，不是个人智慧所能完全理解，因此他开始寻找这份秘密。在崇宁五年的冬天，当宋徽宗还未登基的时候，薛道光在眉县青镇的一座佛寺中听讲遇到了石泰，这位八十岁高龄的石泰不仅神气非凡，而且夜晚还会做针线活计。
当薛道光了解到石泰并非普通人物时，他就故意引用张伯端的一首诗来试探对方。石泰对此感到惊讶，然后询问如何知道张紫阳？原来石泰就是张紫阳的大弟子。当薛道光进一步请教《悟真篇》中的内容时，石泰看出了他的诚挚，便简要地向他解释了大要。这一番交流，让薛道光豁然开朗，一些之前困惑的问题都迎刃而解，就像桶底脱空一般清晰。
从此之后，薛道光信仰上了石泰是真正继承者，并且请求成为他的门下弟子。尽管有可能面临叛教指控，但薛 道光坚定地认为，没有勇敢追求生命和死后的目标，那么自己就是自己的最大阻碍。stone finally agreed to transmit the secret teachings to him, and instructed him to seek out powerful patrons in order to support his spiritual pursuits.
After receiving the transmission, Selian renounced his life as a monk and adopted a layman's appearance. He mixed with the common people while still maintaining his inner light. Eventually, he achieved enlightenment during the reign of Emperor Lizong in 1191 at the age of 114. He left behind several works including "The Song of Danxu", "The Return of Danxu", and annotations on "True Enlightenment". His writings helped spread the ideas of southern internal alchemy.
Selian was a key figure in southern internal alchemy, known for his profound understanding of its principles and practices. His legacy continues to inspire seekers of truth to this day.
