Does the scientific method tell you this? It couldn’t have, because this is a philosophical belief not a scientific one. Very likely, you were indoctrinated to believe this.
Not all “doctrine” (teaching) is wrong and not all indoctrination is wrong. But everyone undergoes some form of indoctrination.
This isn't bad. I don't think it is worth you while to research cannibalism, or a number of other things that people have done/believed in over time. Even if you come to the "right conclusion" there is just too much too research to look into everything people have come up with. My life is worse because I - a non-doctor - had to research all the anti-vaccination claims to see if they really were baseless (they were, but I know from history experts are not always right and once in a while a conspiracy really does occur)
Of course it’s not. Indoctrination is encouraging belief and adherence without question, the scientific method is to only accept after rigorously questioning. It’s trivially easy to apply that recursively. One who questions the scientific method is then by definition not indoctrinated and has not reached their belief via indoctrination.
If you're thinking critically about it (i.e. testing it, questioning its predictions and assumptions, et cetera), it's not indoctrination. If it is, then you've broadened the definition of indoctrination to be equivalent to thought, which is a useless overgeneralization.
And to be clear, you can be religious and not be indoctrinated.
No, you are shown how the ancient Greeks did things, and how they had a lot of ridiculous ideas. Then they introduced Galileo Galilei that showed how experiments can rat out many bad ideas and used them to disprove many of the things written by the ancient Greeks.
That is how we know the scientific method is valuable, nobody has to indoctrinate you to it you can see it yourself. The reason the scientific method is so popular is because you don't have to get indoctrinated into it, it is so obvious that it is a great method.
The problem in all cases is that the criticism and question is more apparent than real, and it does not extend to a whole lot of shared assumptions.
Questioning deeply rooted social norms will get you thrown out of most social groups. Well, questioning social norms held by that group will generally get you thrown out. Questioning deeply held social norms of a different group that the group you're in doesn't like will get you elevated in status, but you aren't actually questioning that group's social norms.
Indoctrination is not a given and should never be acceptable in education.
What is indoctrination? Here's one dictionary definition:
Note "accept it without criticism or question". This is the polar opposite of what education should be, which is to encourage investigation, researching topics with a scientific method instead of believing without question.