1. All living organisms are composed of one or more cells. This means that cells are the fundamental building blocks of all life, from the simplest bacteria to the most complex animals.

2. Cells are the basic unit of structure and function in living organisms. This means that cells are not only the building blocks, but they also carry out all the essential processes of life, such as metabolism, growth, and reproduction.

3. All cells arise from pre-existing cells. This means that cells don't spontaneously appear, but instead are always generated by the division of existing cells.

This theory is fundamental to our understanding of biology and forms the foundation for many other scientific discoveries.