Immanuel Kant 
Moral Law: Groundwork of the Metaphysics of Morals [PDF ebook] 

First published in 2012. Kant’s Groundwork of the Metaphysic of Morals is one of the small books which are truly great: it has exercised on human thought an influence almost ludicrously disproportionate to its size. In moral philosophy it ranks with the ‘Republic’ of Plato and the ‘Ethics’ of Aristotle; and perhaps- partly no doubt through the spread of Christian ideals and through the long experience of the human race during the last two thousand years-it shows in some respects a deeper insight even than these. Its main topic-the supreme principle of morality-is of the utmost importance to all who are not indifferent to the struggle of good against evil. Written, as it was, towards the end of the eighteenth century, it is couched in terms other than those that would be used today; but its message was never more needed than it is at present, when a somewhat arid empiricism is the prevailing fashion in philosophy.