to form or to be formed

[Mike12345]

He is a bright person. A more practical plan began____(form) in his mind.

A to form B to be formed

I think the answer to this question is to be formed. Because a plan can not form itself. But the correct answer given in the book is to form. I got really confused. Could you please tell me which one is correct? Thank you!

 

[tedmc]

Things that are formed in the mind, like plans, ideas and concepts, go through a natural, spontaneous process which is not within our control. This is distinct from physical things which are formed by people through deliberate efforts.

Hence, we say that the former "is formed"(on its own) while the latter is "to be formed"(by somebody's efforts).

 

[Matthew Wai]

a plan can not form itself

'A plan formed in my head.'—quoted from http://www.oxfordlearnersdictionaries.com/definition/american_english/form_2