The idea that all living things composed of cells come from pre-existing cells is called the Cell Theory.

Here's a breakdown:

* Cell Theory: This fundamental theory in biology states that:

* All living organisms are composed of one or more cells.

* The cell is the basic unit of structure and organization in organisms.

* All cells arise from pre-existing cells.

This theory was developed over time by scientists like Robert Hooke, Antonie van Leeuwenhoek, Matthias Schleiden, Theodor Schwann, and Rudolph Virchow. It revolutionized our understanding of life and is considered one of the most important principles in biology.