Paul Broks looks at the philosopher Ludwig Wittgenstein and the problem of "other minds". How do I know you are not a zombie who behaves like a human but actually has no consciousness? Even if you are conscious, how can I tell that what I experience as red, you do not experience as blue? I know what's going on in my own mind, but I can never have direct access to what's going on in yours.

Such questions have troubled philosophers for centuries, but Wittgenstein thought that most of these tough problems were caused by nothing more than a "bewitchment by language". He didn't claim to be able to solve them; rather, he invented a method which he thought of as a kind of philosophical therapy that would cause the problems to melt away. The aim, he said, was to "show the way out of the fly bottle". In the case of the "other minds" problem, he imagined trying to invent a "private language" to describe one's own private mental states, and then showed (he thought) that such an idea was incoherent.

Is the fly out of the fly bottle? Paul Broks suspects not, and psychologist Nicholas Humphrey argues that philosophy took a disastrous turn in the 20th century when it started focusing on language. Humphrey argues that the privacy of our individual minds is a stark and unpalatable fact about human existence which has driven much of our culture.

Presenter: Paul Broks Producer: Jolyon Jenkins.