[ '.'] So I've been thinking lately: Can we know anything with absolute certainty?

We live in a world of concepts. We subscribe to these concepts based on our perceptions. I look at a chair and I realize it is not a cat. Why? Because I have been taught that the chair has certain properties - properties different than a cat. Concepts also change over time. Also, if a concept loses it's meaning, I find no purpose in using it. Thus, anything is only certain depending how it is being viewed today, and if it has meaning as I perceive it.