I am a democrat only on principle, not by instinct -- nobody is that. Doubtless some people say they are, but this world is grievously given to lying.

Summary

This quote suggests that the speaker identifies as a democrat based on principles rather than an innate instinct. They imply that claiming to be politically aligned based solely on instinct is not genuine, as people have a tendency to deceive. In other words, they believe that aligning with a particular political ideology should be a conscious choice driven by rational principles rather than a mere gut feeling.