「氣學」是晚近中國哲學史家提出來的一個概念，它意味一支可以和程朱理學以及陸王心學並列的近代儒家派別。此派別雖長期受到忽略，但仍是中國傳統哲學中少數可以和唯物論哲學接軌的學派。本文認為這種提法有意義，但提出者往往混淆了兩種類型的氣學，一是先天型的，一是後天型的。先天型的氣論雖然都將氣提升到足已和理學的「理」並列的理論高度，但它們所說的氣都是形上之氣，都可視為本體的作用，這是種體驗哲學的用法。相反的，後天型的氣論是自然主義的用法，而且這種氣論是建立在有限人性論上的自然哲學的分支。兩種氣論因為同樣反對朱子的理氣二分以及佛老的「空」、「無」形上學，所以借用了「氣」的論點，但兩者仍貌合神離，分屬於完全不同的思想陣營。本文列舉劉宗周、羅欽順以及張載為例，他們三人分別代表三種先天型氣論的類別，但同樣建立在無限人性論的基礎上。先天型氣論仍屬於理學的陣營，後天型氣學則已拋棄了天道性命相貫通的典範，另覓出一種新的出路。

Qixue 氣學 is a school that can compete with Lixue 理學 and Xinxue 心學 in the works of modem historians of Chinese philosophy. Although neglected for a long time, it remains one of the few schools that can be connected to materialism. Qixue is a significant concept, but those who introduce it often confuse its xiantian mode 先天型 with its houtian mode 後天型. In the xiantian mode, qi 氣 is regarded as the action of the substance, and thus belongs to the category of metaphysics. On the contrary, qi is a materialistic term in the houtian mode, which is a branch of natural philosophy based on the theory of finite human nature. Both of these two modes borrowed the word qi to refute Zhu Xi’s 朱熹 division of li 理 and qi, and the metaphysics of kong 空 and wu 無 of Buddhism and Taoism. This article takes Liu Zongzhou 劉宗周, Luo Qinshun 羅欽順, and Zhang Zai 張載 as examples. They represented three kinds of xiantian mode thought respectively, but all based their philosophies on the theory of infinite human nature. The xiantian mode is still a school within Lixue; the houtian mode looked for a new way to develop.