1. All living organisms are composed of one or more cells. This means that cells are the fundamental building blocks of life, and that no organism can exist without them.

2. The cell is the basic unit of structure and function in living organisms. This means that cells carry out all the necessary life processes, such as metabolism, growth, and reproduction.

3. All cells arise from pre-existing cells. This means that cells do not spontaneously generate, but rather come from the division of other cells. This principle is also known as "biogenesis."