This is one of the fundamental principles of biology, known as the cell theory.

Here's why:

* Cells are the basic unit of life: They carry out all the essential functions necessary for life, such as metabolism, growth, reproduction, and response to stimuli.

* All living organisms are made up of one or more cells: Even the simplest organisms, like bacteria, are composed of single cells. More complex organisms, like plants and animals, are made up of trillions of cells.

* All cells arise from pre-existing cells: New cells are created through cell division, where existing cells divide to produce two daughter cells.

Therefore, everything that is alive, from the smallest bacterium to the largest whale, is composed of cells.