Rush was interested in Native American health. He wanted to find out why Native Americans were susceptible to certain illnesses and whether they suffered from higher mortality rates as compared to other people. Other questions that he raised was whether or not they dreamed more or if their hair turned gray as they got older. His fascination with these people came from his interest in the theory that social scientists can better study the history of their own civilization by studying cultures in earlier stages of development, "primitive men". In his autobiography, he writes "From a review of the three different species of settlers, it appears that there are certain regular stages which mark the progress from the savage to civilized life. The first settler is nearly related to an Indian in his manners. In the second, the Indian manners are more diluted. It is in the third species only that we behold civilization completed. It is to the third species of settlers only that it is proper to apply the term of farmers. While we record the voices of the first and second settlers, it is but just to mention their virtues likewise. Their mutual wants to produce mutual dependence; hence they are kind and friendly to each other. Their solitary situation makes visitors agreeable to them; hence they are hospitable to a stranger."