Do not believe in anything simply because you have heard it.
Do not believe in anything simply because it is spoken and rumored by many.
Do not believe in anything simply because it is found written in your religious books.
Do not believe in anything merely on the authority of your teachers and elders.
Do not believe in traditions because they have been handed down for many generations.
But after observation and analysis, when you find that anything agrees with reason and is conducive to the good and benefit of one and all, then accept it and live up to it.
Beyond describing a way to live and a way to sort out which of the messages we receive that we should take to heart, the above fits a significant part of my definition of critical thinking. For me, life requires that no message of any kind - from anyone- be taken simply on face value without examining its whole.
I read someone's words a while back, lamenting and questioning if they or anyone else had original thoughts and wondering if maybe being too connected (to information) made this worse (don't remember exactly where, sorry, though perhaps many can probably relate to the thought in one fashion or another- if you know someone with a good post on this, let me know). I have had a similar thought about original thinking, though not with the despair that the particular young writer felt.
I think that often we come to similar conclusions about truths* we try to grasp or opinions we form. A thought may be original to us, but if we have been down a similar path as someone else in searching for answers, and our process has paralleled someone else's, we may come to a similar conclusion. We think we have found a revelation, but then discover that someone else found what sounds like the same one, and perhaps stated it with more clarity than we might have if attempting to explain. This doesn't make the concept we have discovered or the opinion we have formed any less important. It doesn't even make it non-original or derivative exactly. And it doesn't mean that we aren't unique, which is where I think the some of the angst of the person I was reading was coming from. What it means is that by examining the various sources we came to the same or similar conclusion as someone else- no more, no less. And it could mean that we might have stumbled upon something that has a larger relevance, maybe even something approximating a truth for our particular set of values.
If we read the same sources, the same histories, the same research, and live in similar cultures to others (with or without examining these influences on our lives), we are bound to often "find" similar truths. Even without the mass of information available to us, we might ponder the same questions and come up with some of the same answers. Perhaps more important than original thought (as in being the first), is to know the influences on where we start and to always evaluate the things we think or find with our critical eye, never accepting "truth" exactly as it is presented by another or the automatic path we travel for our own thoughts, until we have examined it from many angles, and by being as objectively honest as possible with ourselves. It doesn't matter if someone else has been there first. It only matters if we find something to be true for us.
*(the reality of truth- objective or subjective- is a tangent I won't follow this moment)