Kant argued in the Critique of Pure Reason (1781) that we know only appearances, shaped by the forms of space, time and the categories, and never things as they are in themselves. He denied human beings the intellectual intuition that could know them. The mystics claim just such a knowing, and later Idealists tried to overcome the limit.
Thing-in-Itself
Kant’s Ding an sich: reality as it is apart from our ways of knowing it, which we cannot know.
Also: Ding an sich, noumenon