Timon, a celebrated disciple of Pyrrho, added another and, from the speculative point of view, a much more powerful argument, which, however, may equally have been borrowed from the master’s lectures. Readers of the _Posterior Analytics_ will remember how strongly Aristotle dwells on the necessity of starting with first principles which are self-evidently true. The chain of demonstration must have something to hang on, it cannot be carried back _ad infinitum_. Now, Timon would not admit of such a thing as first principles. Every assumption, he says, must rest on some previous assumption, and as this process cannot be continued for ever, there can be no demonstration at all. This became a very favourite weapon with the later Sceptics, and, still at the suggestion of Aristotle, they added the further ‘trope’ of compelling their adversaries to choose between going back _ad infinitum_ and reasoning in a circle—in other words, proving the premises by means of the conclusion. Modern science would not feel much appalled by the sceptical dilemma. Its actual first principles are only provisionally assumed as ultimate, and it is impossible for us to tell how much farther their analysis may be pursued; while, again, their validity is guaranteed by the circular process of showing that the consequences deduced from them agree with the facts of experience. But as against those modern philosophers who, in adherence to the Aristotelian tradition, still seek to base their systems on first principles independent of any individual experience, the sceptical argument is unanswerable, and has even been strengthened by the progress of knowledge. To this day, thinkers of different schools cannot agree about the foundations of belief, and what to one seems self-evidently true, is to another either conceivably or actually false. To Mr. Herbert Spencer the persistence of force is a necessary truth; to Prof. Stanley Jevons its creation is a perfectly possible contingency; while to others, again, the whole conception of force, as understood by Mr. Spencer, is so absolutely unmeaning that they would decline to entertain any proposition about the invariability of the objective reality which it is supposed to represent. And when the _à priori_ dogmatist affects to treat the negations of his opponents as something that they do not think, but only think they think, they may, with perfect fairness, attribute his rejection of their beliefs—as, for example, free-will—to a similar subjective illusion. Moreover, the pure experimentalists can point to a circumstance not foreseen by the ancient sceptics, which is that propositions once generally regarded as incontrovertible by thinking men, are now as generally abandoned by them.