It states that cells are the logical units of the living world, just as atoms are the units of the physical world. The strong version of the cell theory, in other words, declares that life does not exist without cells, and represents therefore a definition of life itself: life is the state of activity of cells and of cellular systems.
The very first problem of biology, the question “What is Life?”, becomes therefore “What is the cell?”. In order to answer this, however, we must recall the answers that have been given in the past to the question “What is a living organism?”.

Mechanism

There are at least two good reasons for saying that modern biology was born in Europe in the first half of the 17th century. One is the discovery of the new world of micro-organisms. The other is the formulation of the first great paradigm of biology: the idea that every living organism is a machine.
This concept – known as mechanism – found in René Descartes its most outstanding speaker, but in reality it was the result of a collective convergence of ideas by scholars of many European cities. From antiquity up to the end of the Renaissance, machines had been built with the sole purpose of obtaining practical benefits, but in the 17th century this view was enlarged by two fundamental novelties.
The first is that machines started to be seen not only as a means for changing the world, but also as an instrument for studying it. In order to look into a microscope, and accept the reality of micro-organisms, one must first of all believe in what is seeing, have faith that the instrument is not producing optical illusions (as some were saying) but is revealing structures that do exist in the real world.
The second novelty of the 17th century is that machines became not only an instrument of knowledge but also a model of knowledge. The idea was developed that to understand the human body it is necessary to divide it into parts, and to study the functioning of its smaller components, just as we do with machines.