When I heard that my friend Martin Anderson had died over the weekend, I gave thanks for his extraordinarily inquisitive mind and the critical role that an institution can play in advancing the cause of a free society. Martin was an economist and political scientist. Stanford University’s Hoover Institution, where he was a senior fellow for more than 40 years, gave him the freedom to explore big policy ideas, and later to discover the hidden depths of a remarkable president— Ronald Reagan.

Martin, who was 78 when he died, was trained as an engineer at Dartmouth and earned a doctorate from the MIT Sloan School of Management in 1962. His doctoral dissertation, “The Federal Bulldozer,” a best seller when published as a book in 1964, destroyed the myth of effective federal urban-renewal programs.

Martin was one of the youngest tenured faculty members in the history of Columbia University before joining candidate Richard Nixon in 1967 as domestic-policy adviser, and then becoming special assistant to President Nixon for his first term.

From my first meeting with him almost 50 years ago, Martin was a bright and engaging thinker. He thought about weighty questions that others had ignored. At the height of the Vietnam War, with all eligible male citizens subject to the military draft, Martin, himself an Army veteran, asked whether the U.S. would be better without conscription. His inside-the-White House arguments, combined with the report of the Gates Commission, and with the endorsement of Nixon’s Defense Secretary Melvin Laird, laid the groundwork for the abolition of the draft in 1973 and today’s all-volunteer U.S. military.

The trendy phrase “outside the box” could have been invented to describe Martin. A quick review of his books: “Conscription,” “Welfare,” “Imposters in the Temple” and his history of the Reagan era, “Revolution,” provides a glimpse at his seminal thinking on a host of policy issues that continue to challenge us today.

He spoke of welfare dependency and the bureaucracies of universities long before these were popular subjects. Ever the thinker, Martin never seemed to be comfortable as a staffer in the executive branch. Yet he urged President Nixon to cut marginal tax rates, to rein in federal spending and to decrease federal regulation. Alas, these policies were not implemented—not until Martin’s next tour in the White House under Ronald Reagan.

In the 1980s, as one of President Reagan’s key economic advisers, Martin was able to coordinate his unwavering belief in the free economy with specific policy recommendations that would change the economic course of the nation, including, perhaps most notably, passage of the Kemp-Roth tax cuts of 1981.

His early relationship with collaborator Richard Allen, who advised on national security for both Nixon and Reagan, led to a further broadening of Martin’s interests, which led to his service as a member of Reagan’s Foreign Intelligence Advisory Board and President George H.W. Bush’s Committee on Arms Control for five years.

But it was Martin Anderson the presidential historian who uncovered the hidden depths of his last real boss. In the 1990s he expressed his concerns to friends, including me, of how misunderstood and underrated President Reagan had been. A tireless researcher, Martin pored over manuscripts of Reagan’s radio broadcasts and his newspaper columns. He also examined the drafts of Reagan’s speeches and read thousands of his personal letters.

This led to a new phase in Martin’s career: uncovering the real Ronald Reagan and reintroducing him to the world. This wasn’t Reagan the docile, cue-card-reading ex-actor, as political opponents and media pundits had unjustly labeled him. But Reagan the careful thinker, principled analyst and realistic assessor of the prospects for American renewal.

With the best-selling 2001 book, “Reagan, In His Own Hand,” Martin, together with his wife, Annelise, and Kiron K. Skinner, reminded Americans of the true depth of Reagan’s understanding and the substance of Reagan as the author of his own ideas. This was followed by “Reagan: A Life in Letters” in 2003, an equally revealing portrait of the man.

Martin and Annelise’s final book, “Ronald Reagan: Decisions of Greatness,” will be published next month by the Hoover Institution Press. As longtime Reagan aide and confidant Ed Meese has said of him, Martin was a loyal and energetic Reaganaut. For all of this, and for so much more, we owe him a tremendous debt.

 - Mr. Feulner is the founder and former president of the Heritage Foundation.