How did Kant’s ideas lead to skepticism about knowledge beyond experience?

Immanuel Kant’s skepticism regarding knowledge beyond experience stems from his doctrine of transcendental idealism, which posits that human knowledge is constructed through the interaction of sense impressions and the mind’s innate structures.1

Kant argued that the human mind imposes specific a priori forms—such as space, time, and the categories of the understanding (e.g., causality)—onto sensory data. Consequently, we can only ever know the world as it appears to us, known as phenomena.234

Because these cognitive structures are the necessary conditions for any possible experience, they also serve as absolute boundaries for theoretical knowledge. Kant maintained that there exists a realm of things-in-themselves, or noumena, which exist independently of our minds. However, since these entities do not exist within space and time and are not subject to causal laws, they remain fundamentally unknowable to human reason. Therefore, any attempt to use pure reason to gain knowledge of metaphysical concepts—such as God, the soul, or absolute freedom—is doomed to failure, as these concepts lie beyond the reach of empirical experience.2135