Hello!
If I understood this correctly, the answer is
"I made it a rule to forbear all direct
![\sm contradiction](https://img.qammunity.org/2019/formulas/english/college/ax5g43dez9ffhq9rbcsqv6hp7v1j18nhyl.png)
to the sentiments of others"
He refrained from directly contradicting other people's opinions.
"When another asserted something that I thought an error, I denied myself the pleasure of contradicting him abruptly, and of showing immediately some absurdity in his proposition;"
Although he wanted to contradict the man, he refrained from doing so. At least, not abruptly.