Thus, after more than fifteen years of personal reflection aided by direct observation, Jefferson came to formulate very clearly in his own mind a certain number of principles founded on reason and verified by facts. Whether he was at that time under the influence of any particular philosopher cannot be proved satisfactorily. It may even be said that it is very improbable, for he was not a man "to submit the whole system of his opinions to the creed of any party of men whatever." Elements of different origins can be recognized in his political philosophy: the theory of natural rights was perhaps Lockian in its principle, but it had been developed by many philosophers, incorporated in the Virginia Bill of Rights and thus naturalized as American even before the Declaration of Independence. The theory of the social compact, too, may have come from Locke; certainly it did not come from Rousseau; but Jefferson introduced into it a fundamental modification when he distinguished between real natural rights and the civil rights guaranteed by society but limited in order to provide for more safety. At any rate, Jefferson's conception of the social compact was far more rigorous, precise, and specific than any that had been proposed before. A man who had been trained as a lawyer knew exactly what a contract was, and how necessary it is, in such an instrument, to write clauses safeguarding both parties. The Bill of Rights was to serve that very purpose: it was nothing but a document enumerating, defining, and recognizing once for all a certain number of rights that every individual specifically reserved in joining a new society. The constitution on the contrary was purely an instrument of government, susceptible of all sorts of amendments from time to time, and certainly from generation to generation. Public opinion was set up as a court of last resort in all cases; for public opinion, not necessarily right in all cases, is always right ultimately in a nation where people have received a minimum of education and are kept informed by a free press. Such were the essential lines of Jefferson's political philosophy on the eve of his departure from France. It does not appear that there was in it anything particularly English or particularly French, although the remote source of some ideas may be traced to English and French political thinkers. His principles, as a matter of fact, belonged to the common fund of political thought drawn upon by all the liberal thinkers of the eighteenth century, and Jefferson, calling no man his master, simply reflected the general trend of his time. But whatever may have been the primary origin of some of his ideas, he was fully convinced that they corresponded to conditions existing in America and nowhere else on earth, that in America alone were they susceptible of immediate application and extensive development.