The Jews had a tradition that stated that angels gave the law to Moses. Moses, in turn, passed it onto Joshua; Joshua passed it onto the elders; the elders passed it onto the prophets; and the prophets passed it onto those who led the synagogue, i.e., the Pharisees and the scribes. So, because the law originated with Moses, the position of being a teacher of God's law became known as Moses's eat. In fact in the synagogue, there was a raised platform at the front. On that platform was a special seat, reserved for the most respected teacher in the synagogue. Today, we speak in universities of the chair of philosophy or the chair of history. The chair is reserved for the most esteemed person in any given subject. Well, Moses' seat was, in essence, the chair of philosophy in ancient Israel. So Jesus is attacking the highest symbol of religious authority. Well, that's what the Pharisees and scribes claimed. He actually wasn't attacking the chair; rather, he was attacking those who occupied the chair. He was saying they were unworthy of the chair.