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Between Gayā and the Place of Enlightenment the Ājīvaka Upaka saw me on the road and said: ‘Friend, your faculties are clear, the colour of your skin is pure and bright. Under whom have you gone forth, friend? Who is your teacher? Whose Dhamma do you profess? I replied to the Ājīvaka Upaka in stanzas:
I am one who has transcended all, a knower of all, Unsullied among all things, renouncing all, By craving’s ceasing freed. Having known this all For myself, to whom should I point as teacher?
I have no teacher, and one like me Exists nowhere in all the world With all its gods, because I have No person for my counterpart.
I am the Accomplished One in the world, I am the Teacher Supreme. I alone am a Fully Enlightened One Whose fires are quenched and extinguished.
I go now to the city of Kāsi To set in motion the Wheel of Dhamma. In a world that has become blind I go to beat the drum of the Deathless. - Ariyapariyesana Sutta
Having seen that there is no being in all the worlds who can be his counterpart let alone be his teacher, the Buddha appointed the Dhamma he realized as his teacher.
Upvote:-3
Māra, the demon king, Māra, the afflicted being. Māra was the Buddha's greatest teacher.
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The histories note that the Buddha spent time with one or two teachers prior to his recognition that they were inadequate. His Dhamma is unique in its form but he did not invent the truth and it is no coincidence that his teachings are in such close agreement with the Upanishads or are counted as an instance of the 'Perennial' philosophy.
But the only proper teacher is acquaintance with truth and so it is the Dhamma that is the true teacher, as the Buddha proposes.