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Why did Kant argue that physics contains synthetic a priori knowledge?

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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 physics contains synthetic a priori knowledge because he believed that scientific principles must be both necessary and universal, yet also informative about the real world.1

For Kant, a judgment is synthetic if the predicate adds information not already contained within the subject, and it is a priori if it is known independently of experience. He maintained that physics relies on specific concepts called categories—such as substance and causality—that are not derived from experience but are instead imposed by the human mind onto the data of experience.123

These categories function as the necessary framework for knowledge, prescribing the general form that the order of experience must take. By "reading" these categories into experience, the mind constitutes the objects of scientific study, allowing for knowledge that is both grounded in reality and necessarily true. This perspective was central to his "Copernican revolution" in philosophy, where he proposed that objects must conform to the mind's a priori structures rather than the mind passively conforming to external things.13