adverb
presumably; I think; in my opinion
Sentence-initial adverb expressing the speaker's considered opinion or conjecture. Often used to introduce a tentative conclusion or personal view.
思うに、彼はもう来ないだろう。
I think he probably won't come anymore.
思うに、この計画は失敗に終わるかもしれない。
In my opinion, this plan might end in failure.
Derived from the verb 思う (to think) in its continuative form 思い, with the particle に, forming an adverbial phrase meaning 'upon thinking'.