What a wee little part of a person's life are his acts and his words! His real life is led in his head, and is known to none but himself. All day long, and every day, the mill of his brain is grinding, and his thoughts, not those other things, are his history. His acts and his words are merely the visible, thin crust of his world, with its scattered snow summits and its vacant wastes of water - and they are so trifling a part of his bulk! a mere skin enveloping it. The mass of him is hidden - it and its volcanic fires that toss and boil, and never rest, night, nor day. These are his life, and they are not written, and cannot be written. Every day would make a whole book of eighty thousand words - three hundred and sixty-five books a year. Biographies are but the clothes and buttons of the man - the biography of the man himself cannot be written.
Every individual is at once the beneficiary and the victim of the linguistic tradition into which he has been born - the beneficiary inasmuch as language gives access to the accumulated records of other people's experience, the victim in so far as it confirms him in the belief that reduced awareness is the only awareness and as it bedevils his sense of reality, so that he is all too apt to take his concepts for data, his words for actual things.
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.