His favourite subject at school was history. He was especially interested in reading about Indians. By the time he was seventeen he made up his mind to write a history of the Indian wars, and this resolution became the controlling power of his life. From that time on he never lost an opportunity of studying Indian life. Nearly all his vacations were spent either with Indian tribes or in carefully going over ground which had been the scenes of many fierce conflicts. About this time Francis Parkman began to have trouble both with his eyes and his heart. He consulted several doctors and travelled extensively, seeking to improve his health. For this reason, and also in order to better acquaint himself with the manners and customs of Indians, he joined a band of Indians on their way to the remote West. During this extensive trip he suffered great pain, yet he knew that it would be unwise to complain or show signs of weakness. Day after day he faced the hardships of a strange life, riding daily on horseback over a wild country, and taking his share with the Indians in hunting buffalo. Instead of improving his health this trip greatly weakened him, and probably did him permanent injury. He could not digest the food that was given him; he became so faint and dizzy that he had to be helped into the saddle, and at times his mind lost its clearness. He could not sleep at nights, and from that time until his death he rarely, if ever, enjoyed a good night's sleep. When he got three hours' sleep out of twenty-four he thought himself lucky. Most of the time he got even less than this. He was attacked with rheumatic gout, which particularly affected one of his knees. This caused him intense suffering, and for many years he could only hobble around with the aid of a stick. Francis Parkman, while still in his early twenties, was a sick man. He had so many disorders that he was never free from pain. Often the pain was so intense that he could not concentrate on any subject. He used to refer to his many troubles as "the enemy", and this can be said to his credit, that he never ceased to fight "the enemy". He had amazing courage; probably not one man in ten thousand could have been so brave and cheerful with so much physical pain. He felt his misfortunes all the more because he so much admired strong men. As a boy, one of the great ambitions of his life had been to become strong physically. One thing which he so much admired about the Indians was their great agility and endurance. And so very early in life, he began to avoid habits which would undermine his strength. For such a man, admiring the physically strong as he did, it became a terrible trial for him to have to go through life as an invalid.