By the end of the Enlightenment epistemology had seemingly reached two alternatives, both of which were viewed as problematic.

The Classical Empiricist Alternative: On the one hand a classical empiricism seemed to point inevitably towards a Humean skepticism with respect to our knowledge of a world beyond impressions and ideas. The scientific reliance on causal connections and inductive reasoning rested on a subjective habit of mind with no (knowable) basis in the objective order of things.

The Kantian Alternative: On the other hand Kant could rescue epistemology from Hume's skeptical attack by having the knowing mind actively synthesize the intuitions of sensibility in a way which presents to the experiencing subject a world of experiences connected by causality. Thus we can have universal and necessary, and so a priori, synthetic knowledge of the world we will experience prior to any experience of it, and Kant claimed to deduce what this knowledge was. However, its object is not the world as it exists "in-itself," but the world as experienced "for a subject." The object thus known is the phenomenal object and it is the only possible object of experience. But -at least according to Kant- the mind must necessarily "posit" an object existing as a "thing-in-itself" (ding-an-sich) apart from our experience of it and in which that experience is said to be "grounded." This is the noumenon or the "noumenal object" implying an object posited by nous other than the object of sensory experience.  Kant thinks it is logically necessary that the mind posit such a noumenon. Without it there would be no grounding for the "intuitions of sensibility" (i.e., sensory percepts) and so our conceptual knowledge would be "empty" or "purely formal." Kant's retention of the thing-in-itself allowed him maintain that his epistemology was realistic; nevertheless, although the mind must posit it, the object as it is apart from our experience remains totally unknowable.