The process of thought, if people consider it at all, is generally believed to be a purely private matter having a momentary bearing on themselves alone.

思想的过程(如果人们能彻底的想一下)通常被认为是一件纯粹的私事，它只对他们自己有短暂的影响。

youdao

Perjury about a purely private matter should at the very least lead this body to question whether no matter how seriously we take the person's violation, for example, of the witness's oath, the drastic remedy of removal from office is the proper response.

CNN: Transcript: White House Counsel Ruff's opening statement

First, America's current-account deficit is no longer a purely private-sector matter.

ECONOMIST: America's current-account deficit