Eileen O'Neill provides an overview of Cavendish's natural philosophy and its critical reception in her introduction to "Observations upon Experimental Philosophy". She describes Cavendish's natural philosophy as rejecting Aristotelianism and mechanical philosophy and favouring Stoic doctrines: while women rarely wrote about natural philosophy in the 17th century, Cavendish published six books on the subject. O'Neill points out that Cavendish herself was not formally educated in natural philosophy, though William Cavendish and his brother Charles shared an interest in the subject and supported her interest and study in the area. She may also have been influenced by social encounters with philosophers such as Thomas Hobbes. O'Neill believes Hobbes (who had instructed Charles in philosophy) had marked influence on Cavendish's natural philosophy, making her one of the few 17th-century supporters of Hobbes' materialist philosophy, which argued that incorporeal souls did not exist in nature. Beginning in the 1660s, Cavendish began to study the work of her contemporaries more seriously. O'Neill suggests that such study was meant to enable Cavendish to argue her own points better by contrast with those of other natural philosophers.