Ilya Prigogine, 86, the Nobel Prize-winning scientist who applied intellect and imagination to some of the most fundamental questions of nature, leading him to be called the "poet of thermodynamics," died May 28 in Brussels. The cause of death was not reported.

In his work, which included research at the University of Texas, Dr. Prigogine offered new ways of approaching and attacking broad questions that have intrigued humans for centuries: such matters as order and chaos, the evolution of the universe and the meaning and direction of time.

He was credited with providing new ways to interpret the second law of thermodynamics, which has been considered a powerful scientific doctrine with applications ranging from explaining chemical reactions to predicting the fate of the universe.

But as Dr. Prigogine explained it in an interview with Omni magazine in 1983, the famous second law takes a mechanistic, deterministic view, which does not always adequately correspond with the complexity and unpredictability of nature.

It applies to systems that have already reached equilibrium, at least in their totality.

But, Dr. Prigogine told interviewer Robert B. Tucker in Omni, "we [on earth] are not going toward equilibrium."

He decided to place more emphasis on evolving systems, where processes occur that do not settle into equilibrium, that dissipate energy and are irreversible.

The irreversible processes, sometimes likened to the squeezing of toothpaste from a tube, provide a means of distinguishing between earlier states and later ones. In that sense, they are sometimes considered, in a metaphorically appealing way, as representing "time's arrow."

The irreversible processes, Dr. Prigogine said in his Nobel lecture, are the ones that lead to the "one-sidedness of time."

Dr. Prigogine, the son of a chemical engineer, was born in Moscow a few months before the 1917 Russian Revolution. Because, he once wrote, his family had a "difficult relationship" with the new government, they fled the country and lived for a time in Germany before moving to Belgium.

He attended secondary school and college in Belgium. Though long fascinated by history, archaeology and philosophy, he majored in chemistry. He received a doctorate in 1941.

He credited a long-standing philosophical interest in the idea of time with prompting his interest in exploring the implications of the second law of thermodynamics and of irreversible, dissipative, nonequilibrium systems.

In biographical material he wrote in connection with receiving the 1977 Nobel Prize in chemistry, he suggested that "maybe the orientation of my work came from the conflict which arose from my humanist vocation as an adolescent and the scientific orientation I chose for my university training."

"Almost by instinct, I turned myself later towards problems of increasing complexity, perhaps in the belief that I could find there a junction in physical science on one hand and in biology and human science on the other."

At Texas, he was a professor of physics and chemical engineering. He received more than 40 honorary degrees.

He was the author or co-author of "Nonequilibrium Statistical Mechanics," "From Being to Becoming: Time and Complexity in the Physical Sciences," "Order Out of Chaos" and "The End of Certainty: Time, Chaos and the New Laws of Nature."

Ilya Prigogine, a Russian native who was educated in Belgium and was a professor at the University of Texas, won the 1977 Nobel chemistry prize.