A theory presents a systematic way of understanding events or situations. It is a set of concepts, definitions, and propositions that explain or predict these events or situations by illustrating the relationships between variables. Theories must be applicable to a broad variety of situations. They are, by nature, abstract, and don’t have a specified content or topic area. Like empty coffee cups, theories have shapes and boundaries, but nothing inside. They become useful when filled with practical topics, goals, and problems.
Adapted from Glanz K, Rimer BK. Theory at a glance: A guide for health promotion practice. NCI, NIH (1995; reprinted 1997)