代词none具体指代了什么

He had a sweet and generous nature, and yet was always blundering; a real feeling for what was beautiful and the capacity to create only what was commonplace; a peculiar delicacy of sentiment and gross manners. He could exercise tact when dealing with the affairs of others, but none when dealing with his own. What a cruel practical joke old Nature played when she flung so many contradictory elements together, and left the man face to face with the perplexing callousness of the universe.

请教老师： but none when dealing with his own. - 这里的none应该是代词吧？问题是具体是如何指代的？

这里的none = but he could not exercise tact 吗？

这里是否可用not或no, 即but not/no when dealing with his own 呢？