A single theme runs through the three essays on St. Thomas gather in this book. It is the theme of mystery or, more exactly, the response of the searching human intellect to the fact of mystery. Both the fact and the response are suggested in a short biography of St. Thomas that forms the first essay and are then sketched out in detail by a presentation of the 'negative element' in his philosophy. The third essay shows that contemporary Existentialism is in basic agreement with the philosophia perennis on this fundamental element of philosophical thinking.