Aquinas was a 13th century university teacher educating students in an ecclesiastical tradition, but because he thought authority without reason could not make sense of truth, he taught questioning. This guide considers the role of regularity and chance in the natural world, mind and matter, freedom and moral obligation, law and society, suffering and evil, hope and hopelessness, and what place can rationally be given to Jesus Christ, to religion and churches, to faith and love, and a God. Extracts are taken from the records of Aquinas’s classroom disputations and two brilliant conspectuses of his teaching: the Summa Contra Gentes attempting a reasoned dialogue with non-Christian (mainly Arabic) scholars, and the Summa Theologiae addressed to his Christian students.