The grammar of the linguist describes, as we have already said, the structure of sentences in the form of a system of rules. The linguist looks at language as such, that is to say, neither under the aspect of producing or speaking nor under the aspect of understanding the particular sentence. Grammar and lexicon are valid for the linguist in the same way for both production and for reception. Therefore, psycholinguistics has also tried for a long time to describe speaking and understanding in one unitary model.