In his book The Scientific Spirit of American Humanism, Stephen P. Weldon (professor of the history of science at the University of Oklahoma) takes on the broad task of elucidating, for both professionals and nonspecialists alike, a treatment of humanism as an intellectual movement in America. Weldon's primary interest is on the role of science in American humanism as both an intellectual and social movement.

Weldon cites as “the central thread” of his book the task of tracing “the trajectory of the visionary outlook I have called the scientific spirit of American humanism”—a spirit “tied to broad human concerns” (11). His aim, however, is not only to assess science's role in humanism but also to show its relation to broader aspects of American culture in the twentieth century—especially the role scientific spirit played in relation to the contors of liberalism's broad tradition in America.

Weldon emphasizes that humanism arose out of America's liberal religion and, therefore, from its very origins in America, conveyed moral and ethical meaning. However, secular humanists viewed humanism as “a way of life” rather than seeing it in religious terms. Liberal Christianity held the concept of “natural theology”—a view that the natural world provided a way of understanding God along with God's revelation in the Bible. The more radical voices, such as those of Thomas Paine and his fellow deists, dispensed with any form of divine revelation other than what is revealed in nature. Indeed, this eighteenth-century “Age of Reason” bore disdain for Christianity. This attitude entered religious circles with Unitarianism which, in the nineteenth century, became more rationalistic and secular by way of William Ellery Channing, Ralph Waldo Emerson, and later with the “higher criticism” of the German tradition. Increasingly radical forms came with University of Chicago's William Rainey Harper, Shailer Matthews, and Germany's Friedrich Strauss. While these religious humanists abandoned supernaturalism, they still talked about God as immanent in nature.

Because of its association with religion, humanism's early battles necessarily centered on conflict with traditional Christian religion's varied institutions and all forms of supernaturalism. Once free of religion an emphasis on science was embraced by humanists. However, conflict soon emerged within humanism over the place of science within the movement itself. The conflicts centered on differences as to how science related to human nature and society. What was involved here were controversies over different naturalistic philosophies then in vogue. Those differences were between a humanistic, value-laden tradition stemming from John Dewey and the dominant Anglo-American analytic tradition. Whereas religious humanists believed God to be discernable in nature, the more radical naturalists saw nature as devoid of any meaning or inherent purpose.

The Humanist Manifesto, appearing in 1933, sought to forge a humanist understanding of religion in the age of science. Adamant that religion must conform to science, it saw nature as self-existing rather than created and asserted that human beings emerged as a continuous process of the natural evolution. All forms of transcendence involving supernaturalism were rejected. John Dewey's 1934 book A Common Faith offered a defense of a humanistic religion.

Weldon provides the reader with a summary treatment of how value-centered humanists dealt with science. He cites the early period as one dominated by ministers and philosophers associated with Unitarianism and the influence of John Dewey. He characterizes a second period coming at the middle of the twentieth century which was more secular—one in which leadership within humanism shifted away from religious to scientific figures. More recently, the period between the early 1960s to the late 1980s is described as one in which a more diverse leadership, including lawyers and secular advocates, sought influence along with philosophers from the positivist and analytic traditions and prominent scientists. The positivist and analytic traditions were reductionist and provided no opportunity for value-oriented humanism.

Weldon then proceeds to trace the ongoing history of value-based humanism through its professionalization via developments at Columbia University with Roy Wood Sellars and the University of Wisconsin with Max Otto, moving on to the 1940 Conference on Science, Philosophy and Religion whose religious tone and resurgence of more traditional voices, such as that of Reinhold Niebuhr, offended many humanists, including Sidney Hook, causing a conflict within humanism as to whether or not it should or should not be considered a religion.

A more secular form of humanism evolved, with many humanists coming from the scientific world and fewer from the religious tradition. With this development, Weldon traces humanists’ shift toward a more scientific-based humanism. This scientific spirit led to rejection of certainty and an acceptance of tentativeness, tolerance, and flexibility. This newfound acceptance of science led to a more international form of humanism. It also led to attention given to issues such as eugenics, race, and population problems.

Weldon then focuses on Priscilla Robertson's role as editor of the Humanist in the mid-1950s for helping understand problems in ethics. Within a decade it shifted from an emphasis on reason to that of personal experience, turning to psychologists such as Carl Rogers, Erich Fromm, and Abraham Maslow. Weldon then records the renewed appeal of fundamentalism in religion, how humanism now battled “creationism,” religious pseudoscience, evolution, and the role of science in the humanist tradition.