For Leon Kass, life truly is one big picture. He calls himself a “humanist— — perhaps because there is no other term broad enough to describe his diverse body of work, but most likely because the driving force behind his professional journey, as he puts it, is a deeply rooted desire to understand what it means to exist as a human being.

Kass has been selected by the National Endowment for the Humanities to deliver this year’s Jefferson Lecture, the most distinguished event on the NEH’s annual agenda. The lecture will take place at 7 tonight at the Warner Theatre. Kass follows a long line of iconic American intellectuals — including Arthur Miller and Toni Morrison — who have delivered the lecture since its inception in 1972.

“It seems to me that the humanities are fundamentally the attempt, by means of human reason and the works of the imagination, to understand as deeply as possible the meaning of our humanity and the human condition,— he said.

Despite his profound interest in the philosophy of humanity, Kass has spent much of his career in the field of bioethics. In 2001, President George W. Bush appointed him head of the President’s Council on Bioethics, where he served until 2005.

During his tenure, Kass was the focus of controversy for his opposition to euthanasia and embryonic stem-cell research. In 2003, the National Review noted that many of his colleagues had become frustrated by Kass’ inclination toward what they called “human exceptionalism— — the assumption that human life is supreme simply because it is human.

Kass will not focus the lecture on his bioethical work, but he expounded on the issue during an interview.

“The world would be a much poorer place if humanity were to disappear than if cockroaches and even if some lovely species of mammals were to disappear,— Kass said. “That has something to do with fact that the human alone among the animals knows that he’s here — can contemplate the beauty of the world, can feel gratitude for his own presence. The animals have worthy lives, but they do not live by choice. They do not have moral responsibility.—

When asked about Kass’ more controversial views, Carole Watson, acting chairwoman of the NEH, noted that groundbreakers like Kass often present opinions that draw criticism from colleagues.

“Over the course of a long career, great public intellectuals are likely to generate ideas that may promote disagreement,— she said in a statement. “This is to be expected, and does not in any way diminish their contributions to the discussion of issues of public concern.—

Kass’ lecture is titled “Looking for an Honest Man: Reflections of an Unlicensed Humanist.— The catalyst for Kass’ lifelong quest, he said, occurred in the summer of 1965, when he and his wife traveled to Mississippi to do civil rights work. During his time among the impoverished, uneducated people of the rural south, he said he discovered an element of human honor and dignity that he hadn’t found in the affluent, college-educated society that reared him.

“It was a very moving experience, living with poor farmers who were uneducated but with a remarkable sense of honor and decency,— he said.

This experience, which he plans to discuss in his lecture, posed a very important question about the relationship between moral and intellectual progress that led him to his journey.

After his tenure as head of the Council on Bioethics ended in 2005, Kass significantly broadened the scope of his work, focusing on humanism as a confluence of scientific, artistic and philosophical concepts.

“The humanities are truth-seeking and truth-disclosing activities — sometimes through history, sometimes through philosophy, sometimes through poetry and fiction,— he said. “Some humanists do this in the large, some in the small. But this is why it is we have esteemed these activities.—

The diversity in Kass’ extensive body of work shows that his interest lies not necessarily in mastering a particular subject, but in understanding the intersection of many subjects and what the nature of that intersection says about the human condition.

His excellence in bridging the gap between the sciences and humanities is what made Kass the best candidate to deliver the Jefferson Lecture, Watson said.

She believes Kass, for all the complexity inherent in his studies, has the unique potential to affect his audience in a powerful but profoundly simple way.

“I would want them to be impressed with the steadiness of the quest in this man’s life to bring the [sciences and humanities] together,— Watson said. “I hope they would then see the value in finding the answers to the most basic questions about what it means to be human.—