Penguin's commercial motivation was, as ever, populist; rendering the classics in an approachable modern English was therefore a difficult task whose execution did not always satisfy the critics. Dr Rieu said of his work that "I have done my best to make Homer easy reading for those who are unfamiliar with the Greek world." He was joined in 1959 by Betty Radice who was first his assistant then, after his retirement in 1964, she assumed the role of joint editor with Robert Baldick. As the publisher's focus changed from the needs of the marketplace to those of the classroom the criticism became more acute, Thomas Gould wrote of the series "most of the philosophical volumes in the Penguin series are bad – some very bad indeed. Since Plato and Aristotle are the most read philosophers in the world today, and since some of these Penguin translations are favourites among professional philosophers in several countries, this amounts to a minor crisis in the history of philosophy."