No one has ever given more eloquent expression to this truth than John Stuart Mill, who was as keen as an observer as he was profound as a thinker. Writing on the subject under discussion, he does not hesitate to say: "Hardly anything can be of greater value to a man of theory and speculation who employs himself, not in collecting materials of knowledge by observation, but in working them up by processes of thought into comprehensive truths of science and laws of conduct, than to carry on his speculations in the companionship and under the criticism of a really superior woman. There is nothing comparable to it for keeping his thoughts within the limits of real things and the actual facts of nature. A woman seldom runs wild after an abstraction.... Women's thoughts are thus as useful in giving reality to those of thinking men as men's thoughts in giving width and largeness to those of women. In depth, as distinguished from breadth, I greatly doubt if even now women, compared with men, are at any disadvantage."[253] We have already learned, from his own avowal, how much Mill was beholden to his wife for her active cooperation in the production of those works of his which have exerted so profound an influence on many phases of modern thought. A more striking illustration of the value of woman's assistance, but in the domain of biology, is found in the biography of the late Professor Huxley. By those who know this distinguished man of science--so remarkable for his intellectual vigor--only from his writings, the impression would be gleaned that he was one of the most independent of thinkers, and that his utterances on all subjects were absolutely personal and entirely unmodified by suggestion or criticism from any quarter. How far this view is from being correct is found in the statement by his son that his father "invariably submitted his writings to the criticism of his wife before they were seen by any other eye. To her judgment was due the toning down of many a passage which erred by excess of vigor, and the clearing up of phrases which would be obscure to the public. In fact, if any essay met with her approval, he felt sure it would not fail of its effect when published."[254] She was not only his "help and stay for forty years; in his struggles ready to counsel, in adversity to comfort," but, over and above this, she was "the critic whose judgment he valued above almost any, and whose praise he cared most to win"--the other self who made his life work possible.[255] An intelligent, sympathetic pair of this kind--and this, as we have seen, is but one of a multitude which illuminates and beautifies the history of science--are competent to achieve wonders. They are like "the two-celled heart beating with one full stroke"-- "Two plummets dropt for one to sound the abyss Of science, and the secrets of the mind."