Old Major's main idea is to make the animals aware of a transformative vision of reality.  His primary purpose was to suggest to the animals that they can be the agents of their own state of being on the farm.  The vision Old Major proposes is one that calls for the overthrow of the humans that own the control of the farm.  In his vision, Old Major seeks to inspire the animals to no longer be relegated to the periphery of decision making, but actually move to the center.  In teaching the animals the song, "Beasts of England, Old Major reaffirms to the animals that they can collectivize to cause change.  This is something that his "disciples" like Snowball and Napoleon take to heart in the acting upon the call for revolution.  Old Major's main idea is one that serves as the catalyst for the events that follow and the advancement of the plot of the novel.