thing-in-itself, 物自身

 thing-in-itself, 物自身

if we know things through our sensory systems and interpret the signal by our brain, then we cannot know the thing-in-itself, because the thing has been interpreted by another system.

some physicists say that we cannot understand a thing, once we measure it, it is not itself. 

Plato has a similar proposition that we cannot only see the shadow of things but not the thing itself.