Like Alexander, Arridaeus was the son of Philip II.  He was born a couple of years before his brother, to Philips’ first wife, a Greek woman named Philinna.  Macedonian custom allowed for a man to have more than one wife and the year after Arridaeus’ birth the king married Olympias, the daughter of a foreign king with whom he wished to cement an alliance.  One year after her marriage Olympias gave birth to her own son, Alexander, and from the very start there can be little doubt that she intended to make him Philips’ heir.

By the time he reached his late teens Arridaeus was widely thought unfit to succeed his father as he was considered weak-willed and feeble-minded.  The truth is that Arridaeus was a highly-intelligent, studious youth whose problem appears to have been epilepsy.   Displaying signs of military genius, Alexander was clearly a more suitable successor and Philip had officially named him as heir.  However, in 338 BC Philip divorced Olympias and she and Alexander were forced to leave the court.  Philip did not disinherit Alexander, but it may only have been a matter of time.  It appears that Olympias’ enemies argued that under supervision Arridaeus was still capable of becoming king.