Thucydides was a Greek historian who lived in the 5th Century BC, and is regarded as a "Father of History" alongside Herodotus - although I confess that before I listened to the In Our Time programme about him I had never heard of him. I think he's been seen as more of a "historian's historian", whereas Herodotus is more of a "popular historian". The programme also told me that Thucydides's work is still important in the field of international relations.