[Footnote 568: This generic principle, viewed under different relations, gives-- 1st. _The principle of Substance_--every quality supposes a subject or real being. 2d. _The principle of Causality_--every thing which begins to be must have a cause. 3d. _The principle of Law_--every phenomenon must obey some uniform law. 4th. _The principle of Final Cause_--every means supposes an end, every existence has a purpose or reason why. 5th. _The principle of Unity_--all plurality supposes a unity as its basis and ground.] [Footnote 569: Cudworth's "Intellectual System," vol. ii. p. 161.] [Footnote 570: "Timæus," ch. ix.] The first business of Plato's dialectic is to demonstrate that the ground and reason of all existence can not be found in the mere objects of sense, nor in any opinions or judgments founded upon sensation. Principles are only so far "first principles" as they are permanent and unchangeable, depending on neither time, nor place, nor circumstances. But the objects of sense are in ceaseless flux and change; they are "_always becoming_;" they can not be said to have any "_real being_." They are not to-day what they were yesterday, and they will never again be what they are now; consequently all opinions founded on mere phenomena are equally fluctuating and uncertain. Setting out, therefore, from the assumption of the fallaciousness of "_opinion_" it examined the various hypotheses which had been bequeathed by previous schools of philosophy, or were now offered by contemporaneous speculators, and showed they were utterly inadequate to the solution of the problem. This scrutiny consisted in searching for the ground of "contradiction"[571] with regard to each opinion founded on sensation, and showing that opposite views were equally tenable. It inquired on what ground these opinions were maintained, and what consequences flowed therefrom, and it showed that the grounds upon which "opinion" was founded, and the conclusions which were drawn from it, were contradictory, and consequently untrue.[572] "They," the Dialecticians, "examined the opinions of men as if they were error; and bringing them together by a reasoning process to the same point, they placed them by the side of each other: and by so placing, they showed that _the opinions are at one and the same time contrary to themselves, about the same things, with reference to the same circumstances, and according to the same premises_."[573] And inasmuch as the same attribute can not, at the same time, be affirmed and denied of the same subject,[574] therefore a thing can not be at once "changeable" and "unchangeable," "movable" and "immovable," "generated" and "eternal."[575] The objects of sense, however generalized and classified, can only give the contingent, the relative, and the finite; therefore the permanent ground and sufficient reason of all phenomenal existence can not be found in opinions and judgments founded upon sensation.