  Sometimes he held such study to be hopeless. The key text on psychology is in The Metaphysical Foundations of Natural Science. There Kant tell us that “the empirical doctrine of the soul … must remain even further removed than chemistry from the rank of what may be called a natural science proper” (Ak. IV:471). (In Kant’s defence, there was nothing resembling a single unified theory of chemical reactions in his time.) The contents of introspection, in his terms inner sense, cannot be studied scientifically for at least five reasons.