休謨曾區分有關人的知識為二：一是與「行動」有關的實踐知識，這類經驗知識關乎人的美感品味、情緒好惡、價值衡量，道德感受，是人追求幸福和美德的指導原理。另一是與「理性」有關的理論知識，這類知識關乎人的理解力、判斷，著重於肯定或否定我們所接觸的事物、採取的行動或者行為。休謨雖然深知「行動」在人生實踐上的價值和意義，卻將之建立在沒有「自我同一性」、沒有「自由意志」的因果法則上；雖然他否定該法則的必然性。流風所及，當代的「行動理論」始終陷於行動是「因果的」，還是「無因果」的爭辯。其中以戴維生(Donald Davidson)的「意向行動理論」最具代表性。當代英美哲學的行動理論雖然肯定「行動」是人基於理由、意圖和目的所為，但卻因為肯定「行動」受到「因果」的決定，始終陷於「自然主義機械論」（以休謨之說為本）和「目的論」（以亞里斯多德之說為本）是否相容的爭議。歷程哲學家懷德海(A.N. Whitehead)本諸亞里斯多德的「實現性」概念，突破英國經驗論的傳統，發展出「現行單元說」(theory of actual entities)，或可作為化解爭議的一個線索。

David Hume once distinguished two types of human knowledge, one is concerned with action, as influenced by taste and sentiment, pursuing one object and avoiding another according to the value which the objects seem to possess. The other is concerned with reason and theory, examining the nature of human understanding with no care of cultivating our manners and with the only interest to find principles helping us to evaluate any particular object, action, or behavior. Hume certainly understood the significance of the concept of action in moral practice; however, he founded the concept on a basis of naturalism and causal mechanism which deprives it of its teleological implications. In addition, Hume also denied the notions of personality identity and of free will which ensure action its spontaneity and freedom. Following this Humean tradition, contemporary analytic philosophers occupied themselves with the issues of the causal or non-causal/ teleological explanations of action, the logic of action, the logical grammar of action sentences, etc., but failed to understand the truth meaning of action itself. The present paper is an attempt to find a clue to the solution of the controversy over mechanism and teleology in action theory and to the understanding of the nature of action by introducing Alfred North Whitehead's theory of actual entities, which suggests us to go beyond the opposition of mechanism to teleology and gives us a more extensive picture of the concept of action.