cell theory is based on a series of discoveries. in 1655, robert hooke coined the word "cell" from his investigations of cork cells. almost two hundred years later, schleiden and schwann proposed the cell theory that states that:
all life forms are made from one or more cells.
the cell is the smallest form of life.

in 1859 louis pasteur's experimental data provided the data to add a third element to the theory:
all cells come from other living cells.
pasteur's work shows us that theories
a) can be disproved by peer review.
b) must be modified as time goes by.
c) can be modified with new empirical data.
d) are never changed regardless of empirical evidence.