本文探討模態信念的生成機制。反事實主義（counterfactualism）認為，模態信念是反事實信念生成機制的副產品。文章首先介紹Williamson的反事實主義理論，該理論認為模態信念源於對反事實情境及其後續發展的模擬想像（simulative imagination）。然而，此理論面臨諸多嚴重問題，如無法解釋傾向模態信念，以及無法合理說明模態性與反事實性之間的關係。本文提出了一個全新的反事實主義理論MAC，主張模態性即為反事實性。MAC繼承了反事實主義的優點，並成功避免了目前主流反事實主義所面臨的問題。

This article explores the belief-forming mechanism of modal beliefs. Counterfactualism posits that modal beliefs are a byproduct of the belief-forming mechanism of counterfactual beliefs. The article first introduces Williamson’s brand of counterfactualism, which argues that modal beliefs originate from the simulative imagination of counterfactual scenarios and their subsequent development. However, Williamson’s theory faces several serious issues, such as the inability to explain dispositional modal beliefs and the inability to clarify the relationship between modality and counterfactuality. The article proposes a new theory of counterfactualism, i.e., MAC, which takes modality to be a species of counterfactuality. MAC inherits the advantages of counterfactualism and successfully avoids the problems faced by mainstream counterfactualist theories.