Interstitiel - start by opening one hint at a time to see if it's enough information
If you haven't found the Interstitiel-pillar, it is located at the northern part of the Campusarea near the main entrance. The position is marked by a red dot in the picture below.
The red numbers refer to specific letters in the quotes from the famous people, the letters are written next to. When you have all the letters, you should be able to form a number.
The letters are E, F, I, and V. When put in the right order, the letters spell FIVE.
The answer to this riddle is therefore 5