Bettany begins her journey in the city of Trier on the banks of the Mosel in Germany. It was here on the 5 May 1818 that Karl Marx, one of the most important philosophers of the 20th century was born.

Nietzsche's uncompromising ideas smashed the safe presuppositions and assumptions of religion, morality and science.

Bettany Hughes travels to Vienna on the trail of Sigmund Freud, the father of psychoanalysis.