[andrewflnr]

Sole source, yes, but that doesn't mean you're reliable. For anything more complicated than "sad", and maybe even for that, self-deception and lack of self-knowledge can be issues. For example, have you correctly identified exactly what it is you're sad about? I know I've caught myself believing dumb things about my feelings.

[bena]

Like I said in my post, everything surrounding it is up for debate. But you can't tell me what I'm feeling. If I say, "I'm sad", that's the end of the story. It's not up for debate.

Even if I'm lying about what I'm feeling, there's no way for you to really know. There's always some room for doubt.