Polybius wrote several works, the majority of which are lost. His earliest work was a biography of the Greek statesman Philopoemen; this work was later used as a source by Plutarch when composing his "Parallel Lives", however the original Polybian text is lost. In addition, Polybius wrote an extensive treatise entitled "Tactics", which may have detailed Roman and Greek military tactics. Small parts of this work may survive in his major "Histories", but the work itself is lost, as well. Another missing work was a historical monograph on the events of the Numantine War. The largest Polybian work was, of course, his "Histories", of which only the first five books survive entirely intact, along with a large portion of the sixth book and fragments of the rest. Along with Cato the Elder (234–149 BC), he can be considered one of the founding fathers of Roman historiography.