While embracing trade policies that foster trade liberalization, Taiwan has clear protectionist policies covering its agricultural trade, which combine border measures with domestic support, and are closely modeled on the policies created by the European Union. The idea of multifunctionality of agriculture - and its link to trade policy - has created a normative framework whereby the agricultural markets have to be shielded in order for them to provide non-commodity attributes or public goods. This paper aims to explore the causal power of ideas (liberalization and multifunctionality) in the definition of Taiwan's agricultural trade policy, by analyzing them from the perspective of historical institutionalism, and taking Taiwan as a case study. It is the institutionalization of the idea of multifunctionality that gives it an explanatory power toward understanding the ideational source of protectionism in agricultural trade.