Conduct is three-fourths of life.

Matthew Arnold (1822–1888) English poet and critic

Literature and Dogma (1873)

Arnold makes multiple references to this concept throughout the book, though never quite in that simple a formulation. The earliest is in chapter 1:

Yet surely the difficulty of religion is great enough by itself, if men would but consider it, to satisfy the most voracious appetite for difficulties. It extends to rightness in the whole range of what we call conduct; in three-fourths, therefore, at the very lowest computation, of human life. The only doubt is whether we ought not to make the range of conduct wider still, and to say it is four-fifths of human life, or five-sixths. But it is better to be under the mark than over it; so let us be content with reckoning conduct as three-fourths of human life.

He goes on to mention the notion multiple times through the book.

By conduct Arnold is referring to action, activity, including religion (ritual and good deeds): things driven by the instincts of self-preservation and sensuality/reproduction; what we do versus what we think. He reserved the other one-fourth to aesthetic and intellectual disciplines (arts and science).