The cell theory is a scientific theory that defines the characteristics that all cells should manifest. Cells were first discovered by Robert Hooke in the 17th century. Matthias Schleiden, Theodor Schwann, and Rudolf Virchow are also accredited with being contributors towards the cell theory.
The tree main parts of the cell theory are:
1. All living things are made of one or more cells (so, if the thing is not made of cells, then it is considered to be non-living).
2. The cell is the smallest unit of life
For example, parts of cells called organelles are not considered living. Nor are viruses because they are not made of cells.
3. All cells come from other cells. In other words, "life comes from life". You cannot get something living from something that is non-living.
This is known as biogenesis. It is in opposition to spontaneous generation.