Hume's strong empiricism, as in Hume's fork as well as Hume's problem of induction, was taken as a threat to Newton's theory of motion. Immanuel Kant responded with his Transcendental Idealism in his 1781 "Critique of Pure Reason", where Kant attributed to the mind a causal role in sensory experience by the mind's aligning the environmental input by arranging those sense data into the experience of space and time. Kant thus reasoned existence of the synthetic "a priori"—combining meanings of terms with states of facts, yet known true without experience of the particular instance—replacing the two prongs of Hume's fork with a three-pronged-fork thesis (Kant's pitchfork) and thus saving Newton's law of universal gravitation.