1. All living organisms are composed of one or more cells. This means that everything that's considered alive is either a single cell or a collection of cells.

2. The cell is the basic unit of structure and organization in organisms. This means that cells are the smallest unit that can carry out all the processes necessary for life.

3. All cells arise from pre-existing cells. This means that new cells are not spontaneously generated; they come from the division of existing cells.

This theory is a fundamental principle of biology and has revolutionized our understanding of life.