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 living organism can exist without them.

2. The cell is the basic unit of structure and function in living organisms. This means that cells are not only the building blocks of life, but they are also responsible for carrying out all the essential functions of life, such as metabolism, reproduction, and response to stimuli.

3. All cells arise from pre-existing cells. This means that cells do not spontaneously generate, but rather they are produced from other cells through the process of cell division.

These three tenets form the foundation of our understanding of life and biology.