I think that a lot of social movements, political movements, powerful ones - certainly they form, collectively, an idea of the truth which becomes hard to question. It becomes a dogma, and from the inside it starts to look like common sense.
We can in fact only define a weed, mutatis mutandis, in terms of the well-known definition of dirt - as matter out of place. What we call a weed is in fact merely a plant growing where we do not want it.