That's something that only the mind can think about, that we don't actually have in the world, the empirical world itself.

那仅仅是心灵可以设想的东西,而在现实的经验世界中,却是不可能存在的

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We are able to talk about the fact that two plus one equals three, but it's not as though we ever come across numbers-- number three itself--anywhere in the empirical world.

我们可与去探讨,二加一等于三的这个事实,但是我们不可能有一天真的碰到数字,在现实世界中碰到个叫"数字三"的东西

耶鲁公开课 - 死亡课程节选