Natural Law

Some people claim we can describe human nature and then somehow 'read off' how humans ought to behave ('act naturally!'). Instead, most Catholic philosophers argue that everything we do is done ultimately for a tiny handful of reasons: to preserve our lives, inform ourselves of truth, develop our relationships, glorify God etc. What lies behind these reasons (life, knowledge, friendship etc.) are the most important constituents of any human life: to be happy we must share in these good things. Anyone who has the use of reason will know this.

To make it impossible for self or others to enjoy these 'goods' is a deeply serious matter. Such conduct is against 'moral principles'. Moral principles describe our various duties to cherish the human goods; they include the principle of respect for persons, impartiality, non-violence etc. Most of us don't need to consult weighty moral principles every day, but we all carry around a set of 'everyday' principles derived from the formal moral ones ('a fair day's pay', 'don't hit a man when he's down' etc.).
Thinking about morality in terms of human goods, moral principles and everyday principles is called 'natural law' thinking. It's perhaps the most important aspect of Catholic ethics.