Answer: experts in the field of magic.
Valdes and Cornelius are two characters in the play Doctor Faustus. They are a witchcrafter and a magician respectively, and as such, experts in the field of magic. Faustus describes these characters as "my dearest friends." Although they only appear in Scene 1, they are very significant to the play. They offer Faustus the possibility of collaborating with them. They argue that Faustus is an intelligent, talented and wise man and that if he were to practice magic with them, they would be able to achieve enormous fame and success.