Main idea of ph-novel
Well. There are different ideas different authors in-put into their novels. With their novels they try to reveal that idea to readers, to people of their times and to everyone who is too busy with common stuff to notice the things of true importance. So, it is okay to say that the main idea of philosophic novel is to show people that they are in a lack of something important.
It was common for all the people of all the times to miss something important in their lives; it was common for people fail to notice something really important behind the curtains of their everyday troubles. And those, who were attentive enough to that "something important", tried to give a hand for people and expressed all they saw in their novels.
The main idea of any philosophic novel is the ideas about foundations and sense of this life. It is also the idea of conflicts between people and the world they live in. there is no better way for one to express all the drawbacks of this life than a way of comparison of one man to all other people. That man is a peculiar one and the polarity for characters typical for times.
By means of comparison of that peculiar one to those who are common in their ideas and lives authors reveal the main mental diseases of their age and their nation. It is the conflicts of personal and social, conflicts between foundations and lack of foundations, conflicts between I-have-to and I-want-to, all those conflicts that disturb our minds.