Artemidorus makes only a brief appearance in the play, but an important one. He is a teacher of rhetoric who also knows some of the conspirators, and he has learned (it is not explained exactly how) of the plot against Caesar. He writes a letter of warning to Caesar, telling him to beware of all the chief conspirators, naming them singly. He hopes to be able to hand it to Caesar personally as he passes:
If thou read this, O Caesar, thou mayst live;
If not, the Fates with traitors do contrive. (II.iii.14-15)
Caesar, however, declines to read the letter despite all Artemidorus’ urgings, declaring in the most grandiose manner that he will attend to personal business last of all – ‘what touches us ourself shall be last served’ (III.i.8) - as he has more important matters of State to attend to. Artemidorus’s letter is the last chance for Caesar to save himself; but he haughtily spurns the opportunity, dismissing Artemidorus as ‘mad’ (III.i.10).
Caesar thus gives the conspiracy a massive helping hand by proudly refusing to heed any warnings that he personally receives, from Artemidorus and the soothsayer, and even from his wife; he helps to seal his own fate.