Compare Cicero, De Fato, c. 7-9. Epistol. Fam. ix. 4.] It was thus that the Megaric dialecticians, with that fertility of mind which belonged to the Platonic and Aristotelian century, stirred up many real problems and difficulties connected with logical evidence, and supplied matters for discussion which not only occupied the speculative minds of the next four or five centuries, but have continued in debate down to the present day. [Side-note: Question between Aristotle and Diodôrus depends upon whether universal regularity of sequence be admitted or denied.] The question about the Possible and Impossible, raised between Aristotle and Diodorus, depends upon the larger question, Whether there are universal laws of Nature or not? whether the sequences are, universally and throughout, composed of assemblages of conditions regularly antecedent, and assemblages of events regularly consequent; though from the number and complication of causes, partly co-operating and partly conflicting with each other, we with our limited intelligence are often unable to predict the course of events in each particular situation. Sokrates, Plato, and Aristotle, all maintained that regular sequence of antecedent and consequent was not universal, but partial only:[63] that there were some agencies essentially regular, in which observation of the past afforded ground for predicting the future--other agencies (or the same agencies on different occasions) essentially irregular, in which the observation of the past afforded no such ground. Aristotle admitted a graduation of causes from perfect regularity to perfect irregularity:--1. The Celestial Spheres, with their included bodies or divine persons, which revolved and exercised a great and preponderant influence throughout the Kosmos, with perfect uniformity; having no power of contraries, _i.e._, having no power of doing anything else but what they actually did (having [Greek: e)nergei/a] without [Greek: du/namis]). 2. The four Elements, in which the natural agencies were to a great degree necessary and uniform, but also in a certain degree otherwise--either always or for the most part uniform ([Greek: to\ ô(s e)pi\ to\ polu/])--tending by inherent appetency towards uniformity, but not always attaining it. 3. Besides these there were two other varieties of Causes accidental, or perfectly irregular--Chance and Spontaneity: powers of contraries, or with equal chance of contrary manifestations--essentially capricious, undeterminable, unpredictable.[64] This _Chance_ of Aristotle--with one of two contraries sure to turn up, though you could never tell beforehand which of the two--was a conception analogous to what logicians sometimes call an Indefinite Proposition, or to what some grammarians have reckoned as a special variety of genders called the _doubtful gender_. There were thus positive causes of regularity, and positive causes of irregularity, the co-operation or conflict of which gave the total manifestations of