The mystery of Thomas Aquinas: Why did he leave his 'Summa' unfinished?

Thomas Aquinas's 'Summa Theologiae' is perhaps the most important philosophical/theological work in Christian history. Why didn't the Angelic Doctor finish it?

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Thomas Aquinas , Monte Cassino , Jean Pierre Torrell , San Domenico Church , Summa Theologiae , Passion Sunday , Middle Ages , Holy Spirit , Summa Contra ,