It's not the most interesting physical object in the world, but it's a whole lot more interesting than a piece of chalk.

它虽不是这个世界上最吸引人的物理对象,但它能干的事比一根粉笔能干的有趣多了

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And the reason for positing these things is because we're clearly able to think about these ideas, and yet, we recognize that the ordinary physical world-- although things may participate in them to varying degrees-- we don't actually come across these objects or entities in the physical world.

我们假定这些的原因是,这样我们就能够清楚的,去思考这些理念,目前我们认识到在正常的现实世界中,尽管很多东西或多或少的参与其中,我们并不会在现实世界中,直接面对这些对象或者个体

耶鲁公开课 - 死亡课程节选