[3] In early 1890 MacKaye worked as a patent lawyer with the Census Bureau in Washington, and in April 1891 he became secretary to Nathaniel Southgate Shaler at the Harvard Geological Department, holding for a number of years various minor appointments in that institution.

During the 28 years that he remained affiliated to that company he published several books on ethics, economics and politics, including Americanized Socialism, The Logic of Conduct and, most notably, The Economy of Happiness.

In this highly original but now largely forgotten work, MacKaye attempted to rescue the utilitarianism of Jeremy Bentham from the changes that this doctrine had subsequently undergone as a result of John Stuart Mill's moderating influence.

Contrary to Einstein, MacKaye proposed a dynamic universe, assuming that all space was filled with radiation of super-frequency and super-penetration moving in all directions with the speed of light.

As he claimed in the concluding paragraph of his paper, subsequently published in The Journal of Philosophy,[6] "If the radiation theory is sound, [...] it is plain that Einstein has discovered nothing about time, space, motion or acceleration unknown to the Newtonians, or shown that what they have hitherto assumed about those magnitudes is contrary to any fact in nature."