Three central themes in cognitive psychology are discussed in the document. The dual-coding hypothesis proposes two codes - imaginal and verbal - and two storage systems for processing information. The conceptual-propositional hypothesis represents visual and verbal information as abstract propositions. The functional-equivalency hypothesis suggests imagery and perception are highly similar. The dual-coding hypothesis in particular states that there are two coding systems - a nonverbal imagery process and a verbal symbolic process - and that concrete words and pictures are coded both imaginally and verbally.