Though Kant is sometimes credited with uniting the warring empiricist and rationalist movements of his time, it might be most accurate to say that he united them by arguing that they were both completely wrong.
His reasoning was something like this: Nobody's senses and thoughts are completely free to do anything - they operate on limits. Thus, neither senses alone nor thoughts alone can ever directly exceed these limits and see the world in itself.
Instead, he argued, the two disciplines needed to work together. Rationalists cannot reasonably deny the entire world of the senses - senses are all we have to know anything but our own minds. Thus any rationalist idea which involved anything other than the mind of the person doing the thinking cannot possibly be correct without a bit of empiricism mixed in with it.
Likewise, there are things empiricists must know in order to operate empirically. A rational knowledge of their senses' limitations CAN help them step outside those limitations. Knowledge of history, memory, prediction, and theory keep us from the need to constantly repeat experiments to prove over and over that something is still true if it was true ten seconds ago. Morality and free will are intuitively apparent, yet completely outside of the empirical realm.
All of which are good arguments. So good, in fact, that both groups decided to play nice for a while. I love it when there's a happy ending.
