Not truth and knowledge

I think it is interesting that Locke pints out the complexity of our language and the words we use more in a moral sense up into 34 sections. Maybe this is me trying to connect my other rhetoric class and the background "enlightenment" piece, but isn't that the whole point of rhetoric. That although words have a definitional meaning but when we really think about a tree me might not all see the same tree. I might see a black and white tree, you might see a cartoon tree and your friend might see a oil paint version of the tree. Maybe I am not understanding why he is trying to isolate rhetoric and the definitions we use as a society down into a perfect line.

According to the OED online, rhetoric is "The art of using language effectively so as to persuade or influence others, esp. the exploitation of figures of speech and other compositional techniques to this end; the study of principles and rules to be followed by a speaker or writer striving for eloquence, esp. as formulated by ancient Greek and Roman writers.

So if one can see the words and their concoctions how is this not truth and knowledge? HOw can we understand something, but there later not be a universal understanding? How does a different interoperation make their understanding meaningless or less correct than someone else? And if there is a universal understanding than why is everyone not thinking along the same lines, or the line that Locke is speaking of.