We like to feel we understand the world around us, but do we really?
Concept
If we break the our understanding of a thing down into it's atomic components we meet the idea of a concept. A concept can be something as simple as a colour (e.g. brown) that is generally agreed as that colour – we are taught that the perception of a certain colour has a certain name. More complex concepts are built out of other concepts; the concept of a horse is a category of things that contains a number of concepts.
Read: Putnam, H. (2008) ‘The Meaning of ‘Meaning’’, In Philosophical Papers Mind, Lang v2: Mind, Language and Reality v. 2, Cambridge, Cambridge University Press, pp. 215–71.
It may argue that it is necessary to have language to have concepts.