There are various theories about Booth's motivations. In a letter to his mother, he wrote of his desire to avenge the South. Doris Kearns Goodwin has endorsed the idea that another factor was Booth's rivalry with his well-known older brother, actor Edwin Booth, who was a loyal Unionist. David S. Reynolds believes that, though disagreeing with his cause, Booth greatly admired the daring of abolitionist John Brown; Booth's sister Asia Booth Clarke quoted him as saying, "John Brown was a man inspired, the grandest character of the century!"