How did Buddha discover wisdom without teacher? — There’s two answers: historical and causal.
- Historical: Born a prince, in comfortable way sets on a journey to try to understand the causes of suffering. Some discoveries great scientists did were not because they received it from a teacher, but because they gathered together — through observation — various phenomena that weren’t paid attention in that way previously.
- Causal: in previous lives he received the teachings of emptiness from many other Buddhas. Taking the form of a prince is for inspiring all of us. Oh he is born as a son of a king, and if he didn’t know the wisdom of emptiness then, how can we! So we are not different, and in higher societies they still start from the base!
When he is referred as the teacher:
- It’s because the vision of emptiness is a causal factor for him to become a teacher of the Nirvana for everybody in the future. This causal factor — for him to discover the wisdom of emptiness — gives him that label of teacher to become. That label of “teacher”, at that time it doesn’t make a lot of sense but that later on it does because he is going to teach this subject matter.
- It’s not because he himself wanted to become a teacher.
Quoting as was spoken because I might have noted the paragraph above wrongly:
The teacher shouldn’t be seen as the Buddha himself becoming Buddha and then teaching sentient beings… — no. It’s the causal factor. The initial discovery of the wisdom of emptiness, that was the causal factor for him to become the teacher of the future. So this causal factor was given the label “the teacher” which is — although not fit at that point — the indication that we is going to teach this subject matter: the wisdom of emptiness in the future.
So “teacher” is the causal factor, which doesn’t fit him at that point.