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Why did Kant claim that things-in-themselves cannot be known?

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Immanuel Kant German philosopher Immanuel Kant surrounded by Ouroboros, an emblematic serpent of ancient Egypt and Greece, engraving by John Chapman, published in London, 1812.

Immanuel Kant argued that things-in-themselves, or noumena, cannot be known because human knowledge is fundamentally limited by the structure of the human mind.12

According to Kant’s transcendental idealism, our experience of the world is not a direct reflection of reality as it exists independently. Instead, the mind actively shapes all sensory data by imposing specific, necessary conditions. These conditions include:23

Forms of Sensibility: All experience must be perceived within the frameworks of space and [time].23
Categories of Understanding: The mind organizes sensory input using pure concepts, such as causality and [substance].23

Because we can only perceive objects once they are "clothed" in these mental forms, we only ever encounter phenomena—things as they appear to us. Things-in-themselves, by definition, exist independently of these human frameworks and do not exist in space or time. Consequently, they remain entirely beyond the reach of theoretical or scientific knowledge, serving only as a limit to what we can know.132