David Hume, an influential empiricist, rigorously challenged the traditional understanding of causal relation. He argued that we never empirically perceive a necessary connection between cause and effect. From his premise that all knowledge derives from sensory experience, Hume noted we only observe constant conjunction: event A is regularly followed by event B. We witness events occurring in sequence, but never the inherent "power" or "necessity" that compels one to cause the other.

Hume concluded that the concept of a necessary causal link isn't an observable fact or a logical deduction, but a psychological habit or custom formed in our minds through repeated observations. Our expectation that B will follow A, due to past experience, is a subjective belief, not an objective truth about the world. This radical skepticism threatened the very foundation of objective scientific knowledge.

Immanuel Kant, profoundly impacted by Hume's challenge, sought to restore the certainty of causality and scientific knowledge. Kant's response introduced his transcendental idealism. He concurred with Hume that causality is not derived from empirical experience (a posteriori). However, he fundamentally disagreed that it was merely a subjective habit.

For Kant, causality is an a priori Category of Understanding – an innate, universal structure of the human mind. Our intellect doesn't find causality in raw sensory data; rather, it imposes causality to organize and make sense of experience. Without applying such categories, sensory input would be an incomprehensible jumble. Thus, while we cannot know "things-in-themselves" (the noumenal world), causality is objectively valid for the phenomenal world (the world as it appears to us). Kant therefore asserted causality as a synthetic a priori judgment, both universal and necessary, thereby salvaging objective knowledge within the bounds of human experience.