What is the cell theory?

The cell theory states that the cell is the fundamental component of all life and all organisms are made up of cells. There are three basic principles to the cell theory. First, the cell is the simplest collection of matter that can live. There are diverse forms of life existing as single-celled organisms. More complex organisms, including plants and animals, are multicellular cooperatives  composed of diverse, specialized cells that could not survive for long on their own. Secondly, all cells come from preexisting cells and are related by division to earlier cells that have been modified in various ways during the long evolutionary history of life on earth. Finally, all of the life processes of an organism occur fundamentally at the cellular level.