Morality ultimately rests, not on sense experience or feelings, but on reason. These summaries and problems deal with Kant's Groundwork of the Metaphysic of Morals. The Groundwork Kant's Groundwork defends morality against those who reduce it to self-interest, feelings, or empirical fact. Instead, the supreme moral principle is based on pure reason. Uncovering this principle is the goal of his book.
Ethics gives necessary truths that hold for all rational beings.
These can't be based on empirical facts about humans. Will Kant tries to uncover the principles behind common sense morality. A good will is good in itself, not just for what it produces. Happiness, the satisfaction of all our desires, is too indeterminate to be a workable guide. Good will isn't the sole and complete good, but it's the highest good and the condition of the worthiness to be happy.