The second obiection of Stahl had a stronger basis, and forced mechanists to change the very definition of “living machine”. In the course of the 18th century, in fact, the view that an organism is a mechanical machine, gradually turned into the idea that it is a chemical machine. This smooth change of perspective went hand in hand with the development of a new engine, an apparatus that was exploiting the chemical reactions of combustion to produce mechanical movements. It was the steam engine that brought together the two sciences, and both mechanists and vitalists realized that a chemical machine is not a contraddiction in terms, as it had been thought, but a reality.
The third objection of Stahl, the idea that machines do not suffer, has never been overcome, and even today is a maior obstacle on the road toward artificial life.
Descartes wrote that only human beings suffer because only they have a soul, while animals are merely mimicking the expressions of pain, but nobody took seriously such an extravagance. It became increasingly clear therefore that an organism cannot be a mere mechanical machine, and eventually the concept of chemical machine was universally accepted.
In the 19th century, the study of the steam engine was pushed all the way up to the highest level of theoretical formalism, and culminated with the discovery of the first two laws of thermodynamics: the principle that energy is neither created or distroyed, and the principle that the disorder (or entropy) of any closed system is always on the increase.
This second principle had a particularly traumatic impact, because it appeared to expose an irreducible difference between physics and biology. In any closed physical system disorder is always increasing, while living organisms not only preserve but often increase their internal order.
The standard replay that organism are not closed but open systems is of little comfort, because one needs to understand how they manage to keep their highly organized state. Eventually however the answer was found and came from two hypotheses:
(1) Living organisms must continuouslly exchange matter and energy with the environment (the idea of biological perpetual motion).