The three statements that define all living things in terms of cells are what comprise cell theory. Theodor Schwann published this theory in a book on plant and animal cells in 1839.
There are three statements regarding cells that make up the cell theory published by Schwann. One is that cells are the units of organization, physiology and structure in all living organisms. The second is that new cells are spontaneously generated, much like crystals. Finally, Schwann believed the cell was a distinct entity in addition to a structural building block in living organisms.
Although Schwann published this theory alone, Matthias Schleiden helped formulate the theory by describing the nuclei of plant cells. Schwann felt these cells were similar to the animal cells he had observed, according to the University of Miami Department of Biology website.