An examination of the place of reason in ethics Book

I was assigned this as part of a teaching assistantship, probably to cull portions relevant to some research my boss was doing. A facsimile edition, I didn't much enjoy reading the antiquated script. As regards the views of the author, his postulation of "moral" and other senses on a virtual par with the physical ones seemed a cheap evasion of the ethical problematic I was concerned about. Basically, he affirms that people are naturally moral, a position not too unlike Rousseau's, though they can be perverted. This hardly seemed to address the imaginary interlocutor I concerned myself with, the Hobbesian individualist or modern sociopath who would say that all interest is self-interest or, even worse, such a one as might be imagined by de Sade who would even challenge any prudential sense of self-interest. What does one say to such a person? What arguments can be made which will compel assent?

For what it's worth, the best arguments I'd become familiar with were the deontological with all of their circularity.