What does "child is the father of the man" mean?

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When wordsworth says, Child is the father of the man, what actually he wanted to say. He could say child is the mother of man either too.!!! In his famous poem, I donot remember name, William wordsworth recalls his child hood by seeing a rainbow in the sky and then at some line he says this famous quote. I still want what exact will he be thinking as he said so.