Daniel Callahan, a pioneering bioethicist who grappled with issues presented by medical advances like organ transplants, prenatal diagnoses and artificial respirators, died on July 16 at a hospital in Dobbs Ferry, N.Y. He was 88.

His son David said the cause was chronic obstructive pulmonary disease.

A leading liberal Roman Catholic thinker who eventually left the church, Mr. Callahan co-founded the Hastings Center in Hastings-on-Hudson, north of New York City, in 1969. The center calls itself the first research institute in the world devoted to bioethics, a field that examines moral uncertainties in medicine, health care and the life sciences.

Before he helped establish that field, Mr. Callahan edited Commonweal, the liberal Catholic magazine, from 1961 to 1968. So provocative were his writings that the historian Rodger Van Allen called him “perhaps the most influential Catholic layman of the 1960s.”

Mr. Callahan went on to become a prolific author and wrote or edited 47 books, many of them addressing what he called “human finitude,” the finiteness of life, and the conflicts that arose when modern medicine tried to extend it.