Moreover, while the practical principles are immutable, always capable of giving form to the most varied historical material, the laws are in perpetual flux and change. The particular modes of change, whether by evolution or revolution, do not concern the philosopher, for whom all can be reduced to a angle one: the free will producing a new law under new conditions. Against the perpetual mutability of the laws, due to the contingent and historical character of their content, clashes the concept of an Eternal Code, or Law of Nature (_jus naturale_), which presumes to determine the content and form of the laws, according to abstract reason, once and forever. This conception is due to an error with which we are now familiar, consisting in the transformation of empirical concepts into principals of universal validity. But from this particular error, as from all errors, we must distinguish certain elements of actual and concrete thought which have been historically associated with it. In the attempts to establish a Law of Nature, we shall then recognise either new concrete legislative programs, the new laws appearing as natural and rational by contrast with the old ones, or an attempt to deduce from, and through, juridical concepts, the principles of a philosophy of the practical. The principle of nationality, fighting for realization against the old dynastic law, appears to its defenders as a typical natural right; and Rousseau, when deducing the principles of the _jus naturale_, warns us that he is not dealing with historical truths, but with hypothetical and conventional reasonings, that is, with principles which transcend every particular determination and have not a positive, but an ideal value. We no longer speak of a Law of Nature, but the error which gave rise to that conception is still vigorous in current social and political discussions; every attempt to change legal conditions is always advocated or resisted by an appeal either to natural rights, which are but arbitrary rationalizations of historical contingencies, or to abstract reasons, principles, or ideas, of which the particular laws or institutions are assumed to be the final and necessary expression. But rationality, morality, and naturality, in the sense in which these qualities are predicated of one or another type of laws and institutions, do not belong to any particular historical determination more than to another; they belong only to the spirit of man and to the concrete values that it realizes among the ever-changing conditions of history.