Buddha's teachings can be accepted only through rational inquiry. However, Buddha shows that rationality alone does not lead us to the truth because a person can argue rationally to prove falsehood as a truth. This is because rational arguments can be brought about by wrong assumptions to arrive at wrong conclusions. Therefore, rationality must be used with wisdom, and truthfulness.
Buddhist rationality is based on truths that are undeniable. Hence the Buddha did not have to assume views when teaching the Dharma. When he stated that it is futile to find an answer to a single first cause or creation of ones life, he did not assume a creator or a search for it. One who think wisely about the suffering or unsatisfactory nature a person undergoes will put forth effort to liberate oneself from the misery rather than finding the first cause. If a person argues and states that there is a single first cause, then there must be a cause to that cause also. Then it becomes an infinite regress.
Some who were holding the view of a creator attempts to argue that Buddha was silent about this question of a first cause because he did not deny the existence of such a cause. However, during the Buddha's time in the ancient India also there were other religions that believed in a creator god. Buddha not only showed the lack of correct rationality in such views but also showed how such views came into existence in the world. The prime causes of such views had been lack of abilities to gain insight through the mind. This was clearly shown in Brahmajala Sutta in the collection of long discourses. In this discourse, Buddha taught how some ascetics who had gone to homeless states started recollecting their past declared the existence of creators due to their inability to see the rebirth linking consciousness. The connection between the changing consciousness could not be established by everyone due to the lack of clarity in the mental states. Hence they rationalized their observations using an explanation that a god created them in this life (in the modern world it has become a belief and a faith due to institutionalization). This is an example where rationality has brought about incorrect conclusions.
Another argument we can often hear is taught to small children in attempting to convert them to other religions. As reported, some people approach small children born to Buddhist families and ask them if they like fathers abandoning them. When children answer stating that they do not like it, the next statement of such missionaries is that Prince Siddhartha abandoned his son. Some adults in the modern world attempting to convert children from Buddha Dharma bring such arguments forward. Again we can see the use of rationality based on falsehood. It is falsehood because the Prince Siddhartha renounced the world out of compassion to all the beings. The same compassion he had for his son as well. The truth, "that he showed the way to ultimate happiness to his son (not only his son but also his former wife and many others)" attest the incorrectness of rational arguments brought about misleading others. In this case also, rational arguments are used with falsehood as a basis.
In a similar vein, others rationalize that there are no results of a person's actions, or no rebirth. These views holds the non-deterministic nature of activities. While holding such views, a person may rationally resort to dishonesty, violence and socially harmful behavior. Buddha encouraged rationality unbiased by views (eternal existence after death, no existence after death etc.).
Buddha while using rationality did not steer away from truth and wisdom. Since truth and wisdom are two essential factors that make rationality beneficial.