The progress of the human mind means to Turgot the progress of knowledge. Its history is the history of the growth and spread of science and the arts. Its advance is increased enlightenment of the understanding. From Adam and Eve down to Lewis the Fourteenth, the record of progress is the chronicle of the ever-increasing additions to the sum of what men know, and the accuracy and fulness with which they know. The chief instrument in this enlightenment is the rising up from time to time of some lofty and superior intelligence; for though human character contains everywhere the same principle, yet certain minds are endowed with a peculiar abundance of talent that is refused to others. 'Circumstances develop these superior talents, or leave them buried in obscurity; and from the infinite variety of these circumstances springs the inequality among nations.' The agricultural stage goes immediately before a decisively polished state, because it is then first that there is that surplus of means of subsistence, which allows men of higher capacity the leisure for using it in the acquisition of knowledge, properly so called. One of the greatest steps was the precious invention of writing, and one of the most rapid was the constitution of mathematical knowledge. The sciences that came next matured more slowly, because in mathematics the explorer has only to compare ideas among one another, while in the others he has to test the conformity of ideas to objective facts. Mathematical truths, becoming more numerous every day, and increasingly fruitful in proportion, lead to the development of hypotheses at once more extensive and more exact, and point to new experiments, which in their turn furnish new problems to solve. 'So necessity perfects the instrument; so mathematics finds support in physics, to which it lends its lamp; so all knowledge is bound together; so, notwithstanding the diversity of their advance, all the sciences lend one another mutual aid; and so, by force of feeling a way, of multiplying systems, of exhausting errors, so to speak, the world at length arrives at the knowledge of a vast number of truths.' It might seem as if a prodigious confusion, as of tongues, would arise from so enormous an advance along so many lines. 'The different sciences, originally confined within a few simple notions common to all, can now, after their advance into more extensive and difficult ideas, only be surveyed apart. But an advance, greater still, brings them together again, because that mutual dependence of all truths is discovered, which, while it links them one to another, throws light on one by another.'