descartes meditations on first philosophy descartes meditations descartes meditations hackett descartes meditations on first philosophy cottingham descartes meditations on first philosophy cambridge descartes meditations on first philosophy hackett 978-0872207981 descartes meditations cambridge descartes meditations oxford descartes meditations on first philosophy isbn 978-0-87220-192-7 descartes meditations on first philosophy cambridge university press