Complex sentence: He then returned to Harvard, where he earned his M.A. in 1973 and PhD in 1974; his PhD thesis, "Herbrand Expansions and Reductions of the Decision Problem", was supervised by Burton Dreben.
Split into simpler sentences: He then returned to Harvard, where he earned his M.A. in 1973 and PhD in 1974. Lewis' PhD thesis, "Herbrand Expansions and Reductions of the Decision Problem", was supervised by Burton Dreben.