The Reality of the Moral Law by C.S. Lewis Doodle (BBC Talk 2, Mere Christianity Chapter 3)

This talk became Chapter 3 of Lewis' book 'Mere Christianity' and was called 'The Reality of the [universal] Moral Law'.

You can find the book here: http://www.amazon.com/Mere-Christiani...

Some helps:
(7:22) "You are saying what is true". Unselfishness means looking after the good of others in society. So when you say "Because it (unselfishness) is good for society" you are essentially saying in reply to the question, "Because unselfishness is unselfishness", which is true but not an explanation as to why we ought to be unselfish. Definition is not explanation, and 'what' is not 'why'.

Lewis talks about this issue extensively in ‘The Abolition of Man’ with one example – in the case of war for a good cause:

“...Let us continue to use the previous example [from Chapter 1] —that of death for a good cause—not, of course, because virtue is the only value or martyrdom the only virtue, but because this is the experimentum crucis [crucial experiment] which shows different systems of thought in the clearest light.

Let us suppose that an innovator in values [i.e. the man trying to create a new and improved morality] regards dulce et decorum [‘it is a sweet and seemly things to die defending one’s country’] and ‘greater love hath no man, that a man lay down his life for his friends’ (John 15:13) as mere irrational sentiments which are to be stripped off in order that we may get down to the ‘realistic’ or ‘basic’ ground of this value. Where will he find such a ground?

First of all, he might say that the real value lay in the utility of such sacrifice to the community. ‘Good’, he might say, ‘means what is useful to the community.’ But of course the death of the community is not useful to the community—only the death of some of its members. What is really meant is that the death of some men is useful to other men. That is very true. But on what ground are some men being asked to die for the benefit of others? Every appeal to pride, honour, shame, or love is excluded by hypothesis. To use these would be to return to sentiment and the Innovator’s task is, having cut all that away, to explain to men, in terms of pure reasoning, why they will be well advised to die that others may live. He may say ‘Unless some of us risk death all of us are certain to die.’ But that will be true only in a limited number of cases; and even when it is true it provokes the very reasonable counter question ‘Why should I be one of those who take the risk?’...” (The Abolition of Man, Chapter 2, The Way).

The 1941 BBC Talks were described and ordered differently to the book made of the talks called ‘Mere Christianity’. Here is the original titles and order from the 1941 Radio Times Magazines:

Series Title: ‘Right and Wrong’ – A Clue to the Meaning of the Universe?
Talk 1 - Common Decency (Chapter 1. The Law of Human Nature), 6 August 1941.
Talk 2 – Scientific Law and Moral Law (Chapter 3. The Reality of the Law), 13 August 1941.
Talk 3 – Materialism or Religion (Chapter 4. What Lies Behind the Law), 20 August 1941.
Talk 4 – What do we do about it? (Chapter 5. We Have Cause to Be Uneasy), 27 August 1941.
Talk 5 – Listener’s Questions (Chapter 2. Some Objections), 3 September 1941.