本文旨在探討老子的天道觀與多瑪斯《神學大全》所闡釋的自然法及其形上學基礎。就哲學上而言，老子的「道」表示那能自行開顯的終極實在本身；其次表示能生的根源，如「道生一，一生二，二生三，三生萬物」一語所示；再次，則表示萬物變化所遵循的法則。就此而言，老子往往以「天道」、「天之道」表之，基本上是天地萬物（當然也包括人在內）所遵循的律則。老子的重點在於萬物皆遵循的律則。至於人定的法律，老子往往加以嚴厲批判，如說「法物滋彰，道賊多有」之類。話雖如此，黃老道家以及受道家影響的法家仍然發展出立基於自然律則的人為法理論。多瑪斯的自然法，是人按照天主賦予的理性本性而行動的法律，可見，多瑪斯的自然法重點是人，既包含了實然（to be存有），也包含了應然(ought to be)，兩者相關而有別。就存有面言，自然法說的是「人依其本性所追求的是善」。就應然面言，自然法的原理是：「人應行善避惡」。本文首先要比較天道與自然法的終極根源，探問老子非位格的道與多瑪斯三位一體的天主如何化生或創造萬物，由慷慨或智慧與愛產生了秩序與法律。然後，再就天道與自然法本身進行比較。最後，論及兩者可會通之處，兼及於道教相關概念。

The basic concern of this paper is to compare Laozi's Heavenly Dao and St. Thomas' natural law for the purpose of bringing them to dialogue. Philosophically speaking, Laozi's concept of Dao means first, the ever self-manifesting and creative Ultimate Reality; second, the Origin giving birth to all things; third, the law of becoming of all things in the universe. For this last meaning, usually Laozi uses the term "Heavenly Dao" to represent laws of nature that all beings in the universe, including human beings, must comply with. As to human laws, he takes a critical position, to the point of saying that "The more legal objects multiply, the more robbers and thieves there will be." Nevertheless, under his influence, Huanglao daoists and legalists have developed their philosophy of positive laws.In Comparison, St. Thomas' natural laws are principles of human action according to human rational nature. The emphasis of St. Thomas is put therefore on the rational nature of human beings, which includes both the aspect of "to be" and that of "ought to be," both being closely related yet distinct. Seen from "to be," the natural law says that all human beings desire goodness by their nature; while as their "ought to be," all human beings should do good and avoid doing evil.This comparative study will focus first on how the generosity of Dao's giving birth to all things in Laozi and the wisdom and love of God in St. Thomas become order and law. Then we will compare Laozi's laws of nature, to which human beings should also live accordingly, with techniques inspired from them, and the principles of St. Thomas' natural laws. We will discuss the possible compatibility and dialogue between St. Thomas thoughts, which are essential to Catholicism, and those of Laozi as followed by lluanlao Daoism and Religious Daoism.