憲法保障言論自由，學理上認為這是因為言論有民主自治、真理追求、自我實現與社會安定等多元價值。其中，真理追求的價值日漸受到重視。言論能夠追求真理，當代的憲政詮釋是從觀念市場隱喻出發。觀念市場隱喻的內涵究竟如何？與真理追求的古典思潮是否符合？本文從思想史與觀念史的角度切入，探討觀念市場隱喻與古典思潮在追求真理的論述上有何差異，特別針對美國最高法院的憲政詮釋與 John Milton、John Stuart Mill兩人的古典思潮進行對照與比較，分析其差異。

Constitutions protect the freedom of speech. There are many reasons why freedom of speech is important, one of which is the claim that truth will confute false. The claim was thought to be the inheritance of John Milton and John Stuart Mill. In fact, the contemporary constitutional interpretation argues that truth will emerge in an open marketplace of ideas. What are the contents of the marketplace of ideas theory? How does the marketplace operate? Is the contemporary constitutional interpretation equivalent with the original thoughts of truth-seeking theories addressed by Milton and Mill? This paper analyzed how the U. S. Supreme Court has applied the marketplace of ideas metaphors to interpret the First Amendment. It then compared the ontology, epistemology, and methodology of the marketplace of ideas metaphors with the texts of Milton’s and Mill’s truth-seeking theories.