This is a book unique in structure - a collection of ideas noted on index cards over a period of 40 years.Acclaimed mathematician Lloyd N Trefethen, Professor of Numerical Analysis at Oxford University, has created an intellectual diary, marking the development of his interests and ideas, from his teenage years to the present. These thoughts stand as signposts, directing us through a mind that applies the same scientific discipline and rigor in everyday life as that needed for success in science and academia. Informative and entertaining, Professor Trefethen's Index Cards is a collage of observations of rare clarity, in subjects ranging from astronomy to family life, and from music to politics.The book will be of interest not only to other scientists and mathematicians, but to anyone in the general public interested in discerning how a scientific outlook informs the way we see broader issues in the societies we live in.