There are above all two core ideas concerning the constitution and the limits of cognition that continue to challenge us in Kant’s theory today.
First, Kant wished to settle an old dispute: some philosophers had derived cognition solely from sensory experience (empiricism), others solely from thinking (rationalism). Kant mediates between both positions: cognition arises only when both faculties cooperate. Hence certain things, which according to him cannot be experienced through the senses, such as God or the soul, are also unknowable.
Second, Kant gives the dispute a new turn by pointing to the insurmountable limit of our cognition: we have no direct access to reality. How the “things-in-themselves” – independent of us – appear remains hidden from us. Thus space, time, and the categories of thought such as “cause” or “possibility” are not properties of the “things-in-themselves,” but structures that we ourselves carry into experience – without us they would not exist at all.