Most experts on Persian language and literature believe that colloquialism in Persian began in the contemporary period with the maturation of a new type of fiction, including short stories and novels. This is because it was in the contemporary period that Iranian writers, in dealing with the new situation in Europe, realized that they were facing a different era; and in this new era, they needed new literary forms and, consequently, a new language to express their views. The view of these experts is based on research conducted by European and American researchers observing European history. In the meantime, this issue has not been addressed, What has caused European writers in the modern era to turn to colloquialism is the “realistic” stance they took, unlike their predecessors. In other words, what made colloquialism an inevitable necessity in Europe during the Enlightenment was the realistic stance that was adopted. This is while among the ancient Persian works, we can find a few works that, since they were written from this realistic stance, inevitably used colloquialism. In this article, we will try to explain the position of Abu'l-Fazl Beyhaqi, as the author of the History of Bayhaqi, by citing his own writing and show that he wrote the History of Bayhaqi from a realistic stance; and this stance made the use of colloquialism in this work an inevitable necessity; and we will take this as a preparation for the fact that basically what makes colloquialism an inevitable necessity is the realistic stance.