Final answer:
Paul Fort founded the Théâtre d'Art in Paris in 1890 to challenge the realist principles of André Antoine, championing a more symbolic and anti-realist approach to theatre.
Step-by-step explanation:
The director who founded the Théâtre d'Art in Paris in 1890 as a direct assault upon the realist principles espoused by André Antoine was Paul Fort. This action was a response to the emerging trend of naturalism in theatre, which was influenced by the works of André Antoine and others who were pursuing realism in their productions. Paul Fort's effort to create the Théâtre d'Art represented a movement towards a more symbolic and anti-realist approach to theatre in contrast to the dominant trends of the time.