Why did Kant argue that people cannot know things-in-themselves?

Immanuel Kant argued that human beings cannot know things-in-themselves (or noumena) because human knowledge is fundamentally structured by the mind's own cognitive faculties.12

According to Kant’s transcendental idealism, our experience of the world is not a direct reflection of reality as it exists independently. Instead, for anything to be an object of experience, it must conform to the requirements of human sensibility and rationality. Specifically:13

• Forms of Intuition: All sensory data are filtered through the a priori forms of space and [time], which are imposed by the mind rather than derived from the objects themselves.12
• Categories of Understanding: The mind further organizes these appearances using pure concepts of the understanding, such as causality and substance, which provide an intelligible structure to experience.12

Because we can only perceive objects that are "clothed" in these mental forms, we are restricted to knowing phenomena—things as they appear to us. The thing-in-itself remains beyond this framework, existing outside of space, time, and causal relations, and thus remains fundamentally unknowable to theoretical reason.412