Galileo’s theorem, stating that force is proportional to acceleration, does not come immediately from experience; logically considered, it is a free statement. It comes from the intuitively acquired knowledge that the phenomena of motion can be easily understood if acceleration is regarded as the fundamental phenomenon whose causes are sought. That this is not obvious in itself – to be precise, that it is not necessary – can be seen by looking at the history of mechanics before Galileo. The logical arbitrariness of this point of view is revealed by the fact that the general theory of relativity6 has found it necessary to modify it.