An archaic adjectival (rentaishi) used (especially with negation) to indicate something not particularly notable or significant; rarely used in modern Japanese.
然せるにあらず（させるにあらず）—そう言うほどのことではない。
"Not so" — it is not something to be particularly remarked upon.
それは然せるにあらぬ事なり（古風な表現）。
That is not something noteworthy (archaic expression).