The idea of such a provisional code seems to have originated with Zeno;[61] but the form under which we now know it is the result of at least two successive revisions. The first and most important is due to Panaetius, a Stoic philosopher of the second century B.C., on whose views the study of Plato and Aristotle exercised a considerable influence. A work of this teacher on the _Duties of Man_ furnished Cicero with the materials for his celebrated _De Officiis_, under which form its lessons have passed into the educational literature of modern Europe. The Latin treatise is written in a somewhat frigid and uninteresting style, whether through the fault of Cicero or of his guide we cannot tell. The principles laid down are excellent, but there is no vital bond of union holding them together. We can hardly imagine that the author’s son, for whom the work was originally designed, or anyone else since his time, felt himself much benefited by its perusal. Taken, however, as a register of the height reached by ordinary educated sentiment under the influence of speculative ideas, and of the limits imposed by it in turn on their vagaries, after four centuries of continual interaction, the _De Officiis_ presents us with very satisfactory results. The old quadripartite division of the virtues is reproduced; but each is treated in a large and liberal spirit, marking an immense advance on Aristotle’s definitions, wherever the two can be compared. Wisdom is identified with the investigation of truth; and there is a caution against believing on insufficient evidence, which advantageously contrasts with what were soon to be the lessons of theology on the same subject. The other great intellectual duty inculcated is to refrain from wasting our energies on difficult and useless enquiries.[62] This injunction has been taken up and very impressively repeated by some philosophers in our own time; but in the mouth of Cicero it probably involved much greater restrictions on the study of science than they would be disposed to admit. And the limits now prescribed to speculation by Positivism will perhaps seem not less injudicious, when viewed in the light of future discoveries, than those fixed by the ancient moralists seem to us who know what would have been lost had they always been treated with respect.