Lewis Pirenne is the chairman of Foundation's Board of Trustees and leader of the Encylopedists, meaning that he basically runs the Foundation until the end of "The Encyclopedists."
Pirennes sums his up own mode of operation when he tells Hardin, "[Local politics are] none of our business. We are first of all and last of all—scientists. And our concern is the Encyclopedia" (II.1.23). This make Pirenne the first character—maybe the second, if you count Linge Chen from "The Psychohistorians"—whose butts heads with the protagonist because he just doesn't want to change. The galaxy may be a different place than it was fifty years ago, when he came to the Foundation to complete the Encyclopedia, but he's determined to see his job through.
You have to admire the man for sticking to his guns, right? Well, not so much. Actually, his inability to adapt and change is directly opposed to the science he's believes he is promoting.
See, one difference between science and religion as ways of understanding the world is that scientists have to change their perspective of the world as they gather new evidence and information. They must evolve to meet the new demands and challenges put forth on them by said evidence and information. Not only that, but they must also go out and actually seek this new evidence out firsthand in a little thing called research and experimentation.
Hello, major character flaw. Pirenne can't or won't change his ways to meet the ever-evolving universe, despite the fact that he's surrounded by evidence that things are changing. He's stuck listening to Britney Spears when everyone else has moved on to Lady Gaga. Not very rational if you ask us—and certainly not very scientific of him.
Also, the Encyclopedia is composed of old facts from the First Empire being stored for future generations. That's all well and good, and we here at Shmoop love our history. But Hardin has a good point when he asks Pirenne, "Have you ever thought of working onward, extending [the First Empire's] knowledge and improving upon it? No! You're quite willing to stagnate" (II.3.37).
New information improves science and helps us adapt to a changing word. For the Foundation to survive, it has to keep up. Hardin sees this fact for what it is, but is too infatuated with the old Empire to accept it—even though all his tidy facts are rendered useless as the old Empire fails. The solution? Pirenne has to give up control of the Foundation.