本體論為任何研究之始，能動者 / 結構爭論為以 Waltz 為首之結構現實主義與 Wendt 為代表之建構主義在本體論層次的重要歧異點之一，為此，兩派學者自 1980 年代末黨論戰迄今。儘管本體論的問題具有政治性，不見得有終極與明確的答案，但籍由本體論的討論可以讓吾人更能瞭解兩派理論在深層假設的異同。本文在敘明能動者/結構爭論在理論發展上的重要性後，進行爭論中相關重要概念——如能動者、能動性、結構等——的闡述，除追蹤這些概念在社會學上的根源外，也討論了 Wendt 在國家能動性的論點與「中間道路」在本體論上的立論基礎等幾項建構主義者在概念上引發爭議之處。在能動者/結構爭論中，建構主義者對 Waltz 結構現實主義提出兩項批判: Waltz實乃個體主義者與 Waltz 的結構制約單元的論點忽略能動者的能動性。本文綜整 Waltz 在國家是「單元」、結構的「形成」、結構的「屬性」、結構的「效應」、結構的 「變化」等五項相關論點，本文認為:以結構決定論的標準觀之，Waltz的觀點固然與Levi-Strauss與Wallerstein等厚實的結構主義有段距離，但從Waltz對結構與單元的嚴加切割，及其與Durkheim之間的應合，Waltz的結構現實主義宜視為一薄弱形態的結構主義。儘管如此，Waltz雖做了許多在方法論上向單元層次傾斜的修正意見，但他在本體論上依舊欠缺對能動者的能動性與該能動性可轉型結構的相關認織，結構現實主義與建構主義在能動性上的差異依舊鮮明。 最後，本文認為:基於分析層次表陳所意涵之理論簡潔度的重要性與必然性，即便對於相對複雜性的建構主義，分析層次的表陳依舊是必須面對的挑戰，本文即提出在本體論上能動者/結構的討論範疇內一個能夠凸顯 Waltz 與 Wendt 立場差異性的分析層次表陳。

All research begins with ontology. The agent/structure debate represents one of the most profound discrepancies and the least conclusive contests in this regard between Structural Realism and Constructivism since the late 1980s. Though politics is the terrain of competing ontology；definite and clear-cut answers are hardly attainable, a serious discussion on ontological issues can lead us to penetrate the assumptions deeply embedded in both theories.

This article first examines theoretical significances in the agent/structure debate and next unpacks related concepts -- such as agent, agency, and structure - as well as their origins in sociology. In the process, some conceptual controversies over state agency and ontological core of epistemological via media in Wendt's theorization are also briefly discussed. This article then turns to constructivist's two-front criticism against Waltz's Structural Realism in the agent/structure debate: Waltz is in fact an individualist and his ignorance about agency. After summarized and examined Waltz's arguments in respect of state as “unit,” “emergence" of structure, “property” of structure, structural “effects,” and structural “changes,” this article concludes: though Waltz's theory, in strict terms of structural determinism, does not match with those thick Structuralism proposed by Levi-Strauss or Wallerstein, it is nevertheless better to see Waltz's theory as a thin Structuralism if Waltz's serious efforts to differentiate structure from agent and his theoretical responsiveness to Durkheim are properly appreciated. Waltz later makes many methodological amendments that lead his arguments more unit-level oriented; however, such efforts cannot bridge the existing gap between his Structural Realism and Constructivism since his ontological commitments about agent's agency and its potential to transform the structure are both wanting. Last, the significance and inevitability of theoretical parsimony implied in the illustration of level of analysis are highlighted; so constructivists, who cherish the complexity in their theory, may find unwarranted not to engage such an illustration. Thus, this article presents an illustration for the purpose of demonstrating ontological differences between Waltz and Wendt in the agent/structure debate.