In Kantian philosophy, the thing-in-itself (German: Ding an sich) is the status of objects as they are, independent of representation and observation. The concept of the thing-in-itself was introduced by the German philosopher Immanuel Kant, and over the following centuries was met with controversy among later philosophers. It is closely related to Kant's concept of noumena or the objects of inquiry, as opposed to phenomena, 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.

— Prolegomena, § 32

The first to criticize the concept of a thing-in-itself was F. H. Jacobi, with the expression:

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.

The anonymously published work Aenesidemus was one of the most successful attacks against the project of Kant. According to Kant's teaching, things-in-themselves cannot cause appearances, since the category of causality can only find application to objects of experience. Kant, therefore, does not have the right to claim the existence of things-in-themselves.

This contradiction was subsequently generally accepted as being the main problem of the thing-in-itself. The attack on the thing-in-itself, and the skeptical work in general, had a big impact on Fichte, and Schopenhauer called G. E. Schulze, who was revealed to be the author, “the acutest" of Kant's opponents.