The thing-in-itself (German: Ding an sich) is a concept introduced by Immanuel Kant. Things-in-themselves would be objects as they are independent of observation. The concept led to much controversy among philosophers. It is closely related to Kant's concept of noumenon or the object of inquiry, as opposed to phenomenon, its manifestations.
In his doctrine of transcendental idealism, Kant argued the sum of all objects, the empirical world, is a complex of appearances whose existence and connection occur only in our representations. Kant introduces the thing-in-itself as follows:
And we indeed, rightly considering objects of sense as mere appearances, confess thereby that they are based upon a thing in itself, though we know not this thing as it is in itself, but only know its appearances, viz., the way in which our senses are affected by this unknown something.
I could not enter into the system without the assumption of the concept of the thing-in-itself and, on the other hand, I could not remain in it with this concept.