I'm not entirely sure, but I'm pretty sure that you can be fairly sure that there is absolutely nothing you can be sure of. If you take the time to think about it all, you start to realize that absolutely everything we experience might not even be real. For instance, it is actually possible that our thoughts might not be entirely our own. Our words have no absolute meaning. In example, when someone mentions the color green it can be assumed that everyone would think of a green color but it can also be assumed that they are not all thinking of the same shade of green. Any time you come to a conclusion about something you are probably wrong or at least not entirely correct. Why else would there be so many varying philosophies, religions and social concepts and always more to come. It can be assumed that we can't know everything about everything for sure.
Source: I'm as sure as I can be that this quote came from my own mind through my finger tips and onto the world wide web.
Contributed by: ataraxia