The Theatætus is followed by the trilogy of the Sophist, the Statesman, and the Philosopher, which completes the Megaric group of dialogues. The first of these dialogues examines the conception of appearance, that is of the not-being, the last (for which the Parmenides may be taken) the conception of being. Both dialogues are especially directed to the Eleatic doctrine. After Plato had recognized the conception in its simple unity as that which abides in the change of phenomena, his attention was naturally turned towards the Eleatics, who in an opposite way had attained the similar result that in unity consists all true substantiality, and to multiplicity as such no true being belongs. In order more easily on the one side to carry out this fundamental thought of the Eleatic to its legitimate result, in which the Megarians had already preceded him, he was obliged to give a metaphysical substance to his abstract conceptions of species, _i. e._ ideas. But on the other side, he could not agree with the inflexibility and exclusiveness of the Eleatic unity, unless he would wholly sacrifice the multiplicity of things; he was rather obliged to attempt to show by a dialectic development of the Eleatic principle that the one must be at the same time a totality, organically connected, and embracing multiplicity in itself. This double relation to the Eleatic principle is carried out by the Sophist and the Parmenides; by the former polemically against the Eleatic doctrine, in that it proves the being of the appearance or the not-being, and by the latter pacifically, in that it analyzes the Eleatic one by its own logical consequences into many. The inner progress of the doctrine of Ideas in the Megaric group of dialogues is therefore this, viz., that the Theatætus, in opposition to the Heraclitico-Protagorean theory of the absolute becoming, affirms the objective and independent reality of ideas, and the Sophist shows their reciprocal relation and combining qualities, while the Parmenides in fine exhibits their whole dialectic completeness with their relation to the phenomenal world.