After rejecting the warnings of his wife Calpurnia because Decius flatters him and informs him that the Senate has decided to award Caesar a crown on this day, Caesar's ambition "that should be made of sterner stuff," as Marc Antony later says gets the better of him and he scolds Calpurnia,
How foolish do your fears seem now, Calpurnia!
I am ashamed I did yield to them.
Give me my robe, for I will go. (2.2.109-111)
So, in Scene 3, Artemidorus, a teacher and friend of some of the conspirators has learned of the plan to assassinate Caesar; consequently, he writes Caesar, cautioning him. He stands waiting for Caesar to pass so that he can hand him the earnest warning signed by him, a loving friend. Aloud to himself he says that Caesar will live if he reads this letter; if not, "the Fates with traitors do contrive" (2.3.13-14)
Here Artemidorus has clearly warned Julius Caesar that there are those who entertain treacherous thoughts.against him.