一般來說，反事實條件句是關於沒有實際發生的事件，所以使得人們想知道是否能夠獲得反事實條件句的知識。本文的焦點在於我們能否證成反事實條件句，而不是我們能否知道反事實條件句的真假。本文主張我們應該區分兩種反事實條件句，因為不同種類的反事實條件句會需要不同種類的證成。本文將論證，我們可以通過實際世界的知識或概念分析來證成反事實條件句，儘管我們可能仍無法知道反事實條件句的真假。因此，本文的結論是：我們確實具有反事實條件句的知識，儘管不如我們所認為的那樣多。

Counterfactuals are about events that have never happened, so that people may wonder if we can assess knowledge of counterfactuals. The focus of the present paper is on the issue of whether counterfactuals can be justified, rather than the issue of whether counterfactuals can be known. It is argued that two kinds of counterfactuals should be distinguished, for they may ask for different kinds of justifications. The aim is to argue that we can justify counterfactuals by knowledge of the actual world or conceptual analysis, though in the end we may still fail to have knowledge of counterfactuals. The conclusion is that we do have knowledge of counterfactuals, though not as much as we thought.