Part of the Royal Institute of Philosophy's lecture series The History of Philosophy. Thomas Aquinas bluntly says that we do not know what God is. In this lecture I try to explain what he means by this conclusion and how he arrives at it. Aquinas’s position on human knowledge of God sharply contrasts with that of many authors working in the area of philosophy of religion, so I draw attention to this fact and defend Aquinas to some extent. Filmed on 6th February, 2015