The idea that all living things are made of cells is called the Cell Theory. It's one of the fundamental principles of biology.

Here are the key points of the Cell Theory:

* All living organisms are composed of one or more cells. This means that even the smallest single-celled organisms, like bacteria, are made of cells.

* The cell is the basic unit of life. This means that cells are the smallest units that can perform all the functions necessary for life.

* All cells arise from pre-existing cells. This means that new cells are not spontaneously generated but are created through the division of existing cells.

The Cell Theory was developed over time by several scientists, including Robert Hooke, Antonie van Leeuwenhoek, Matthias Schleiden, and Theodor Schwann. It revolutionized our understanding of life and forms the basis for many other biological discoveries.