Knowledge can be expressed in language using a plethora of grammatical means. Four major groups of meanings related to knowledge are Evidentiality: grammatical expression of information source; Egophoricity: grammatical expression of access to knowledge; Mirativity: grammatical expression of expectation of knowledge; and Epistemic modality: grammatical expression of attitude to knowledge. The four groups of categories interact. Some develop overtones of the others. Evidentials stand apart from other means in many ways, including their correlations with speech genres and social environment. This essay presents a framework which connects the expression of knowledge across the world's languages in a coherent way, showing their dependencies and complexities, and pathways of historical development in various scenarios, including language obsolescence.