History and theory are the salt and pepper of architecture. They can go hand in hand but are two complete different components. History tells what has happened based on facts and chronological events. Theory touches base with a much greater depth of concept. It is true that theory often explains history, and that theories are often based on the events of the past, it is important to acknowledge the difference between the two.
Theory is a belief based on an interpretation of a situation, event, phenomena, or group of facts. Architectural theories justify the reasoning behind the work of an architect. It takes under consideration the rules of behavior, society, the economy, and industry, not just rules of building. Theory is rhetorical, for it is meant to persuade others into accepting the theory as a truth. Theory is based on interpretation of facts, not facts themselves. This is where it differs greatest from history.
Architectural history tells of different eras and styles of architecture. History tells of the technology of the particular time, and of the methods used in the building process. Theory on the other hand tells why things were done the way they were done throughout history. Theory tells why a certain style was believed to be the best for the situation. As theory is the “why” of architecture, history is its “how” and “what”.