In Our Time
They've all been the subject of an episode of In Our Time.
In Our Time is a BBC Radio 4 program, where Melvyn Bragg invites two or three experts in to the studio, where they then spend 45 minutes calmly talking about a subject.
Highly recommended.
The various categories are:
Culture
History
Philosophy
Religion
Science
I have to confess that sometimes I do nod off, but listening to intelligent people talking calmly about intelligent things (often that I've never even heard of before) is a really good way to stimulate the curious mind.