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: www.amazon.com/Mere-Christianity-C-S-Lewis/dp/0060652926
(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'.
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.