I also find it interesting, but I'm questioning its (possibly not-so-obvious) meaning: could it be that you should try to distinguish what you really think from what you merely think (in order to avoid saying spurious things maybe)? I find it often surprisingly hard to determine what I really think or feel..

I don't know why, but I was really taken aback when I realized that there was this other side to "Do not lie." It's not just avoiding telling falsehoods, but also speaking up about important truths as you see them.

could you elaborate on what exactly it means?

I guess it’s hard to pin down exactly :) I guess it’s a variation of ‘To thine own self be true’. For me, I think there tends to be a background process running which attempts to find the correct and socially acceptable thing to say. This is often not related to what I find to be true and important myself. It’s kind of a simulation of a simulation: I-perceive-that-what-I-perceive-are-people’s-intentions-is-answered-by-this-pattern-I-have-observed. It’s a habit and it’s a relief when it can be tuned down a bit.