Berseria is not just about emotion and reason. In studying their relationship, a third variable is found in the concept of will—the faculty with which one makes one's decisions. In essence, an individual's will is the power by which the decision is made to indulge emotion or reason in a given moment. As someone who possesses an intimate understanding of this equation, Eizen is a strong proponent of "taking control of the wheel."
Early on in his development, Laphicet lacks the drive and understanding to make decisions for himself. Eizen imparts unto him the wisdom of steering life the way he chooses, which stands opposite to the Abbey's teachings of treating malakhim like tools. Spurred on by those words, Laphicet begins his journey of self-discovery and eventually becomes his own person after spending his initial years as a doll.
Eizen claims that there's nothing he hates more than those who threaten to impede him on his chosen path. Along the way, he commits a number of deeds that doesn't cast him in the best light, such as torturing a knight for passage and dragging the party into unnecessary fights—and at first glance, his advice seems like one rooted in selfishness, but there's a certain truth to what he says.
At the end of the day, Eizen is interested in following what his heart wants. Semantics are important within the context of Berseria's narrative, as his talk of heart isn't directly associated with emotion. In Japanese, there are two separate words for heart—one that describes the organ, and the other that encompasses the spirit and soul of an individual. In other words, Eizen's philosophy reflects the latter usage of the word, which means that one can stay true to one's heart by choosing to be logical or emotional; what's important that there's the power of choice.
What is Eizen's creed? Free will, the ability to claim a life with meaning.Eizen's creed is truly appropriate and even ironic, considering his fate in Zestiria, where he has lost all sense of who he is. At the same time, he asserts in Berseria that becoming a dragon does not scare him, so long as he goes with the knowledge that he's led a life that reflects who he is up to that point, in the same way that he's embraced his image as the Reaper. Ultimately, he argues that his inevitable descent as a dragon would be the culmination of his current decisions, and that's just fine with him.
In Tales of Zestiria the X, the anime adaptation of the game, adds a twist to the resolution of Eizen's story. In my opinion, the resulting sentiment is sweet, but weakens the worldbuilding and colors the story of the white-horned dragon—Theodora—in Berseria. Still, it's heartwarming to see a branch that offers a different resolution for Eizen.