What's Invincible Ignorance?

Mother reads from The Christian Mother and The Christian Father, and answers questions from:

Blue-Integrity, who asks about invincible ignorance;

Katherine, who is confused about claims regarding the Miracle of the Sun at Fatima;

Alastair, who asks Mother about a book called Limits to Growth;

And Mary, who wonders about what Our Lord said regarding St. John the Baptist.