One of the most enigmatic of Jesus' sayings is found in Mark 4:11-12. In citing Isa 6:9-10 Jesus seems to mean that he uses parables in order to hinder a certain people from understanding, even to the extent of preventing them from repeting and being forgiven by God. For more than a century the same questions have caught the attention of scholars: Did Jesus really utter this saying? Was it not composed by the early Church or by Mark? Did the saying - whether it originated from Jesus or from the Church - originally refer to the hardening function of parables? In order to solve this problem, by means of examining the book of Isaiah and various Jewish writings, the author first seeks to elucidate what idea Isa 6:9-10 could have conveyed to an ancient reader of Isaiah in the context of the Second Temple, Palestinian Judaism. The author then goes on to demonstrate that, against such an ideological background, the unity of the parable of the sower (Mark 4:3-9) and its explanation (4:14-20) is the key to understanding the sayind in question. Thus, the connection of the parable - the saying - the explanation is intelligible in the Palestinian Jewish context of Jesus' time. Therefore, it can be argued that the whole connection is Jesuanic, and not from the early Church or Mark. The author also clarifies the meaning of the saying in the Marcan context and compares it with the meaning in the Jesuanic context. This study sheds light on that part of the dark history of the Jesus tradition to which scholars have not previously paid attention.