Idealism is a philosophical perspective in which we interpret experiences based on the ideal, the perfect, and even the spiritual. We believe that there actually is an ideal state as opposed to interpreting experiences through a realistic, pessimistic viewpoint.
We can actually see Jack London's novel Martin Eden portraying themes of idealism and even questioning idealism. London's protagonist Martin feels he has experienced the ideal the moment he is invited into the upper-middle-class home of the Morses, after rescuing Arthur Morse from bullies. In this home, Martin sees paintings up close for the first time and admires their beauty. He is also treated with respect for the fist time by being addressed by Arthur as Mr. Eden. He even sees a stack of books on a table, and the narrator relays that "into his eyes leaped a wistfulness and a yearning as promptly as the yearning leaps into the eyes of a starving man at sight of food." All of these things help to paint upper-middle-class life in Martin's mind as something perfect, beautiful, and ideal. Through his conversations with Ruth, he even begins to idealize education and the pursuit of a writing career as his tickets out of his working-class life.
However, sadly, though Martin achieves fame at the end, he also resorts to suicide because he sees his fame isn't based on his talent, as would fit his idealism, but is rather based on social trends. Jack London uses Martin's suicide serves to question Martin's idealism, since London was a dedicated realist.