Professor Philippa Foot: Philosopher regarded as being among the finest moral thinkers of the age

The moral philosopher Philippa Foot was among the great pioneering moral philosophers of the age. The modesty of her output – two volumes of collected papers, Virtues and Vices (1978), Moral Dilemmas (2002) and a culminating book, Natural Goodness (2001) – should not belie its superior, often stunning, quality: absolutely "about the stuff", in Cora Diamond's phrase about Foot's close philosophical colleague Elizabeth Anscombe, a philosopher for whose appointment at Somerville Foot proposed vacating her own. That was a gesture characteristic of her lived integrity.

Hers is moral philosophy at its best, never abandoning real life; and she kept words on a Wittgensteinian leash, close to their ordinary homes. Her thinking is driven not by commitment to abstract doctrine but by a haunted preoccupation with cases and examples, such as the German Letter Writers' sacrifice of their lives in protesting against the Third Reich. Foot returned again and again to such situations to explore their moral import.

She was born Philippa Bosanquet in 1920 and educated mainly at home before going to Somerville College, Oxford, in 1939 to study Politics, Philosophy and Economics. She graduated in 1942 and worked as a government economist for the rest of the Second World War before returning to Somerville. In 1949, she became a Fellow of the college. In the 1960s and 1970s she held visiting professorships at Cornell, MIT, Berkeley, and the City University of New York, before settling in 1976 at UCLA, where she stayed 15 years. She was an Honorary Fellow of Somerville until her death.

Her mother had been born in the White House, the daughter of President Grover Cleveland; her father was an industrialist in Kirkleatham, where she was brought up in typical upper-class fashion. She and her sister Marion, hunting with other local grandees, recalled huntsmen on horseback and full rig entering Raby Castle hall. In adult life she would use hunting metaphors to illustrate discussions of philosophers: who fell at the first ditch, who were front-runners. Her gift for philosophy mystified her: believing that she lacked other kinds of intelligence or knowledge, she could not account for it. Yet she knew she was a front-runner all the same.

Around the age of eight Philippa got abdominal tuberculosis and suffered the then "cure" of sleeping for a year, including winter months on an out-of-doors balcony, in North Yorkshire. This taught her a hard-won self-sufficiency. When their mother threatened to sack Nanny, her main source of love and care, the 10-year-old Philippa and her 11-year-old sister Marion packed their infant bags to leave home. The death of this good woman in 1976 was a deep loss to her.

While the product of this background, she felt contempt for its political conservatism and treatment of women. From governesses she never even learnt "which came first, the Romans or the Greeks". When she won a place at Somerville one of her mother's friends said, " Never mind, dear: she doesn't LOOK clever!"

Shortly after the war she married the historian MRD Foot, from the Dorset home of the Pinney family, whose eldest daughter, then a child, remembered Philippa as uniquely attentive to her as a complete human being. Philippa always claimed that Donald MacKinnon at Oxford had taught her and Iris Murdoch virtue, and how properly to apprehend another person: but she had her own virtue, too. Indeed Murdoch, with whom she shared a flat for two years in 1943-45, praised her brilliance and warmth, noting that she was "morally tough and subtle, with lots of will and self-control".

Her marriage failed, but if its childlessness was a source of grief, friendships delighted and consoled her: she was generous, tolerant, patient and wise, with an excellent memory for the detail of other lives. She was also affectionate but unsentimental, bereft of self-pity, and with a keen sense of the ridiculous. Foot's long friendship with Murdoch was possibly even more significant for Murdoch than for Foot. She appears as Paula in Murdoch's novel The Nice and the Good and her and Marion's alliance against their mother made an appearance in A Fairly Honourable Defeat, where Morgan's fear of her sister also echoed Dame Iris's fear of Philippa.

Foot's published work is confined to ethics, though she would occasionally give workshops on Later Wittgenstein. But within ethics she ranged widely: there are papers on special issues, abortion, and euthanasia, on freedom of the will, virtues and vices, a critique of utilitarianism, moral dilemmas. Yet her core concerns were the major themes of morality's objectivity and rationality.

Foot took two contemporary orthodoxies to be central. First, that facts and values are distinct, that arguments about values, unlike those about facts, may break down without rational error (Stevenson, Ayer, Hare). Second, that morality necessarily gives reason to everyone. The first she attacked, devastatingly, in the wonderful papers Moral Arguments and Moral Beliefs (1958). On "the private enterprise" view of evaluative predicates, individuals are in control of relevance – of what facts are, and are not, to count as evaluatively relevant: I am free to dismiss the facts that you bring to argue that X is good, and so our argument may break down. Against this, Foot argued for public criteria. She took a mid-range evaluative predicate, "rude", and argued that if certain facts obtained you couldn't deny that some piece of behaviour was rude, and vice versa, and still count as having linguistic mastery of the predicate. Similarly, you can't call anything you like "good" or "bad" and be understood (e.g. stand on a chair and flap your arms and call that action "good").

However, this brilliant advocacy of an ordinary objectivity in value was accompanied by an assumption of a subjectivity of reason. She thought: surely you only have reason to be courageous if it links to something you happen to care about? If it doesn't, then still to say "you ought to be courageous, etc" is to treat those words as imbued with magical, automatic, reason-giving force. So, if you ought to be virtuous, it is either because this promotes something you care about, or it is itself something you care about. Foot thought that not everyone cared about virtues as ends.

Initially she accepted the second orthodoxy, arguing that everyone cared about their self-interest and that the virtues promoted this. The position is unappealing – and, as she realised, had serious difficulties over justice. This led to the "scandalous" position of Morality as a System of Hypothetical Imperatives (1972) where she abandoned the second orthodoxy: some care about morality and the virtues as ends, others do not – and for the latter there is no reason to be moral.

Foot came to see that the problem was her assumption of the subjectivity of reason. With this obstacle removed, the final picture is revealed: there are objective facts about what is good and bad in human action and life, and these facts ground objective reasons for what a human ought to do (whether they realise it or not); there are objective facts not just about means but about what ends we all ought to have or to care about.

But what are the criteria for "the human good" and what ultimately makes for a good life? Some fuller account was needed. Natural Goodness gives us an objectivity of practical reason together with the kind of factual basis of moral evaluation: the good and bad dispositions of the human will are to be assessed in a basically comparable way to assessing the goodness and badness of a tree's roots (or good and bad health). This book is an appropriate capstone to a wonderful career.

When it came out in 2001, she enjoyed her publisher's joke that "Wittgenstein once said that it was impossible to do philosophy slowly enough. But Philippa" – her book having incubated for decades – "has proved Wittgenstein wrong". It went into several languages. She knew she was a world-class philosopher and was proud that she had a following in, for example, Sweden, Berlin and Bulgaria as well as the US, to all of which she travelled in later years. Foot quotes Wittgenstein's "Be crude and then we shall get on", and she helped us all to get on immeasurably, with exemplary wisdom, tenacity and humanity.

Philippa Ruth Bosanquet, philosopher: born 3 October 1920; Griffin Professor, University of California at Los Angeles 1988–91, then Emeritus, Professor of Philosophy 1974–91; married 1945 Michael Richard Daniell Foot (divorced 1960); died 3 October 2010.