WELL before anyone had heard of her son, Shirley Robin Letwin was well known as a political philosopher who influenced the Conservative intellectual revival in the 1970s and 1980s. She was involved in the Centre for Policy Studies, founded by the late Sir Keith Joseph, and Margaret Thatcher. Dr Letwin, brought up in Chicago and who died in 1993, challenged the collectivism of the postwar era. Her final book, The Anatomy of Thatcherism, discussed the principles that underpinned the Thatcher premiership, praising the “vigorous virtues: upright, self-sufficient, energetic, adventurous, independent-minded, loyal to friends, robust against foes”.