In general the term principle indicates the bases and the initial assumptions on which they are based or a theory is based, and which are, by convention or law recognized the validity, general and universal within the theory itself; then flow from it, therefore, by inference or logical consequence, or in a manner consistent with the principle itself, a whole series of secondary laws (u call laws) that characterize the theory in question, or vice versa, all the laws of the theory must comply with the basic principle of the same to be placed into.

In the experimental sciences generally speaking a principle is unprovable in an absolute sense, but is assumed to be true in virtue of the multiplicity of observations that occur and predictions that it, and the laws derived from it, allow you to do, coming at the same time not, or losing its characteristic of universality, in the presence of even a single experimental evidence that denies it, as is typical of science and its experimental method.

In other contexts, such as in philosophy or theretical phy, a principle is assumed as true as it is in what is recognized as "semantically" correct.

Starting from Aristotele : logic, through Galileo, Cartesio, Bacone... till F. Enriques and G. Peano, Russell

Dear Colleagues, thank you for your replies that are very interesting.

When I would find again the historical background for the difference between the "French" choice and the "English" one, I will let you know.

Many thanks.