You can imagine the perfect chair in your mind but you cannot build the perfect chair.
Or something very similar. I believe that is Kant but I cannot find any reference to the quote anywhere. Any one have some ideas?
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It sounds like Kant's idea of Noumena. For example:
We then realise that not only are the drops of rain mere appearances, but that even their round shape, nay even the space in which they fall, are nothing in themselves, but merely modifications or fundamental forms of our sensible intuition, and that the transcendental object remains unknown to us.
Alternatively, you may be thinking of the Theory of Forms associated with Plato.