"My manner of thinking, so you say, cannot be approved. Do you suppose I care? A poor fool indeed is he who adopts a manner of thinking for others!"

The Marquis de Sade is expressing his contempt for those who attempt to dictate how he ought to think. He is asserting his independence and autonomy, and is not going to comply with the expectations of others. He is making it clear that he will not be swayed by the opinions of others, and that he will believe for himself. He is also implying that those who attempt to manage the ideas of others are absurd. He is stressing the value of specific autonomy and flexibility of thought. He is asserting that nobody needs to be forced to think in a specific method, and that everybody needs to be enabled to think for themselves. He is asserting that it is wrong to attempt to control the ideas of others, which everyone needs to be permitted to think easily.