In 1851 Miss Martineau edited a volume of _Letters on the Laws of Man's Nature and Development_. Its form is that of a correspondence between herself and H. G. Atkinson, and it expounds that doctrine of philosophical atheism to which Miss Martineau in _Eastern Life_ had depicted the course of human belief as tending. The existence of a first cause is not denied, but is declared unknowable, and the authors, while regarded by others as denying it, certainly considered themselves to be affirming the doctrine of man's moral obligation. Atkinson was a zealous exponent of mesmerism, and the prominence given to the topics of mesmerism and clairvoyance heightened the general disapprobation of the book, which caused a lasting division between Miss Martineau and some of her friends.
She published a condensed English version of the _Philosophie Positive_ (1853). To the _Daily News_ she contributed regularly from 1852 to 1866. Her _Letters from Ireland_, written during a visit to that country in the summer of 1852, appeared in that paper. She was for many years a contributor to the _Westminster Review_, and was one of the little band of supporters whose pecuniary assistance in 1854 prevented its extinction or forced sale. In the early part of 1855 Miss Martineau found herself suffering from heart disease. She now began to write her autobiography, but her life, which she supposed to be so near its close, was prolonged for twenty years. She died at "The Knoll" on the 27th of June 1876.
She cultivated a tiny farm at Ambleside with success, and her poorer neighbours owed much to her. Her busy life bears the consistent impress of two leading characteristics--industry and sincerity. The verdict which she records on herself in the autobiographical sketch left to be published by the _Daily News_ has been endorsed by posterity. She says--"Her original power was nothing more than was due to earnestness and intellectual clearness within a certain range. With small imaginative and suggestive powers, and therefore nothing approaching to genius, she could see clearly what she did see, and give a clear expression to what she had to say. In short, she could popularize while she could neither discover nor invent." Her judgment on large questions was clear and sound, and was always the judgment of a mind naturally progressive and Protestant.