"In the words of the Catechism of the Catholic Church, “The entire Law of the Gospel is contained in the new commandment of Jesus, to love one another as he has loved us.” But what does it mean to love someone? As Aquinas put it (citing Aristotle), at a minimum “to love is to will good for someone”; that is, to do what we can to see that a person has a good life. But what is a good life? We might, thinking of the core message of Jesus, say that it’s a life of loving others. But this response just takes us in a circle. Jesus tells us that to lead a good life we should love one another, but loving one another requires helping one another lead good lives. Unless we first know what it is to lead a good life, Jesus’ law of love gives us little guidance on how to live. This is no mere abstract worry. There are many competing conceptions of a good life. Utilitarians like John Stuart Mill think it is one that maximizes the pleasure of mankind as a whole. Others, like Immanuel Kant, think it is a life of virtue for its own sake, even if this requires renouncing pleasure. Followers of Aristotle think it requires flourishing through various intellectual, psychological and social virtues. How a life of love for others should be lived depends on which conception of a good life is correct."
Returning to the Sermon on the Mount - NYTimes.com