Knowledge is divided to knowledge by present and acquired knowledge. In the knowledge by present, unlike of acquired knowledge, the knowledge and the known are confederate. The definition of acquired knowledge is the presence of the image of thing in the mind. It is divided to concept and assertion. There are three groups about the nature of assertion. The first says that the assertion is a complex of judgment and elements of proposition. The second group believe it is the concept along judgment. And finally, assertion is the judgment. But there are differences about the explanation of judgment, i.e. understanding of the proposition's truth. On the other hand, the image of truth is the judgment to the truth of proposition. On author's view, we can obtain the correct definition of assertion, based on the attention to these differences.