同樣地，存在偶然係一個先驗命題。例如，巴黎嘅米原器曾經以一米作為標準。於是就有下述嘅命題，“米原器長一米”，呢個命題只係偶然，因為我咃仍然可以用其他長度定義一米。不過，佢係屬於先驗，因為一米係由米原器嘅長度嚟定義，所以米原器肯定係長一米（喺它重以一米為標準嗰陣時）。但呢個都係一種循環定義。
In the same way, existence is a priori proposition. For example, the rice prototype in Paris used to be based on one meter. So there is the following proposition, "The original meter is one meter long", this proposition is just accidental, because I can still define one meter with other lengths. However, it is a priori, because one meter is defined by the length of the original meter, so the original meter must be one meter long (when it weighs one meter as the standard). But this is also a circular definition.