 The question of scope also arises when it comes to the subject matter of the article, which is empiricism as a doctrine rather than as a practice. From the very beginning of what the Greeks called “the investigation of nature” (hē peri phuseōs historia), there have been those who studied the natural world by means of careful observation. There were medical writers who observed and recorded symptoms, astronomers who observed the movements of the heavens, and students of music who placed their ears “close [to their instruments] as though trying to hear what their neighbours were saying” (Plato Rep. 531a6–7). These people were surely empiricists in practice. But such figures will be of interest to us only if they also reflected on what they were doing, developing second-order beliefs about knowledge. It is these second-order beliefs that will be the focus of the present study.