expression
in (a period of time, manner); at (a time, stage in life); while
Formal/literary expression used to indicate a time, stage, or manner in which something occurs. Often follows nouns or adverbial phrases.
彼は二十歳にして初めて海外に行った。
He went abroad for the first time at the age of twenty.
一瞬にして状況を理解した。
He grasped the situation in an instant.
expression
Formal/literary concessive use, often with a sense of surprise or contrast. Follows nouns or nominalized phrases.
彼は若いにして、とても落ち着いている。
Despite being young, he is very composed.
この問題は簡単にして難しい。
expression
as well as; (both) ... and ...
Formal/literary listing pattern, often in the form AにしてB, meaning 'both A and B' or 'A as well as B'. Used with nouns or adjectival nouns.
彼は学者にして詩人でもある。
He is both a scholar and a poet.
その料理は簡単にして美味だ。
The dish is simple as well as delicious.
expression
Archaic locative use, equivalent to modern で or に. Found in classical texts; not used in modern Japanese.
古語では「山にして遊ぶ」のように場所を表す。
In classical language, it can indicate location, as in 'playing in the mountains'.
This problem, although simple, is difficult.