声には出さない

内心もの凄く驚きながらも、声には出さない。

Hi. What is the function of には in red here? I know には can indicate ability, ownership, judgment, etc. But none of the function I know fits in this context.
Thank you.

That's just the topicalization of 声に出さない. は is the contrastive marker here.

Toritoribe said:

That's just the topicalization of 声に出さない. は is the contrastive marker here.

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Thank you. But I don't know the function of に here. Why be 声に出さない? Why not be 声を出さない?

zuotengdazuo said:

Thank you. But I don't know the function of に here. Why be 声に出さない? Why not be 声を出さない?

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The difference between 声には出さない and 声に出さない is just one of contrast, as Toritoribe-san already mentioned. With the は, it adds an additional sense of the speaker not expressing their surprise aloud (with their voice), at least, though they did feel it inside.

声を出さない and 声に出さない have different nuances. With 声を出す, 声 is the direct object of 出す. It just means to make a sound with one's voice. With 声に出す, 声 is the target of 出す. You're taking something (in this case, the speaker's feeling of surprise) and putting it into sound with one's voice, i.e. vocalizing it.

It's not that 声に出さない and 声を出さない are ungrammatical or impossible, but the writer/speaker chose this particular phrasing because the nuance fits what he's trying to say.

に indicates target/state/result of change here. 声に出す is close to 声にして出す "to output in the state/form of voice", i.e., "to say/express in voice" as a set.

As bentenmusume-san explained perfectly, 声に出す usually used in a situation where the subject feels/thinks something inside them/in their mind, so expressions like 怒りを声に出す or 不満を声に出す can work. には is an emphasized form of it.

On the other hand, in 声を出す, the contents of 声 is not always important. 声 might be meaningless sounds like a groan, for instance, 気絶していて全く声を出さない. 声に出さない doesn't work in this case.

Furthermore, 声に出す also means "to put emotion into voice", thus, 声に出さない is also used in a situation where the speaker says words without the emotion. In your example, the subject might say something, but they didn't show their surprise in their voice. 声を出さない doesn't work in this case, needless to say.

Thank you for your excellent explanation !