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。薛道光本是一位僧侣，以法名紫贤闻名世间，被尊称毗陵禅师。他曾在长安云游，居住于开福寺，与修岩长老及其他高僧深入交流。在一次观察桔槔时，他突然开悟，并以诗句表达了自己的顿悟：“轧轧相从响发时，不从他得豁然知。槔说尽无生曲，井里泥蛇舞柘枝。”这次经历深受二老赞赏，使他进一步理解无上圆明真实法要，并展现出了机锋敏捷和宗学兼通的才华。

随后，他意识到六祖慧能之后的悟性并非一成不变，因此决心寻找黄梅传法中的秘密。他在宋徽宗崇宁五年的冬天，在眉县青镇的一座佛寺中听讲，当时遇到了来自扶风县的八十有五岁的道人石泰。这位神秘人物不仅外貌非凡，而且夜晚还能够做针线活计。薛道光通过引用张伯端诗句，让石泰惊讶，然后又引述《悟真篇》中的内容，请教石泰，这样石泰才相信他的诚意，并简略地向他解释了大要。

此后，薛道光对这些知识产生了深刻理解，就像打开了一扇大门一样，一切疑团都被打破了。他信任石泰是张紫阳真正的弟子，从而请求成为他的门下弟子。尽管如此，对于可能遭受叛教指控的问题，薛 道光仍坚定地回答：对于生命与死亡的大事，如果拘泥于门派之争，那么不是自己自误呢？ stone hear this, nodded repeatedly and said that since his teacher Zhang Purple Yang passed on the teachings, it had been nearly 30 years. Today he finally met someone who could inherit them. He then transmitted the oral tradition and essential secrets to sal ke daung, instructing him to seek out powerful patrons in the secular world for support.

From then on, sal ke daung abandoned his monastic robes and became a lay practitioner of Daoism. He wore simple clothing and lived humbly. During his lifetime, he composed several works such as "Dan Zhi Ge" (Song of Elixir), "Huan Dan Fu Ming Bian" (Recovering Lost Life), and "Wu Zhen Bian Zhuo" (Annotated Version of Awakening True Reality). These writings have contributed significantly to the development of southern school internal alchemy thought.

At the end of his life during Song Dynasty's Shao Xi era in 1191 at age 114, he left behind an epitaph that reads: "Iron horse gallops into the sea / Mud snake soars into heaven / The three islands of Penglai are not located in the west." His legacy continues to inspire generations seeking truth within themselves.