In the Theaetetus, Plato initially proposes, but ultimately rejects, three models of knowledge to explain the conditions for the possibility of philosophic knowledge: perception (151e-187a); true belief (187b-201c); and justified true belief. (201d-210a) Plato first rejects knowledge by perception because if all perceptible things were always changing into something different, and nothing ever remains the same, then it would be impossible to consistently name or judge anything. (182a-183a) Plato also rejects knowledge by true belief because, if all knowledge were true beliefs, then there could be no knowledge of false beliefs, and every belief would be trivially true. (187e) Finally Plato proposes that the truth of any belief may only be discerned through a concept of justification. For the purpose of explaining justification, Socrates distinguishes the thought of the first simple elements that cannot be known from the thought of the final complex combinations that we presume to know in our concepts (logos). (202a) Socrates then proposes a part-whole dilemma: any complex concept is either (i) identical to the unknown combination of simple elements (203d), or (ii) different from this combination by some further addition above the parts (204a). Yet if the concept is (i) identical then it is just as unknown as the simple elements, but if it is (ii) different then any further addition must likewise be either a simple element that is thinkable but unknowable or not a simple element and hence unthinkable. (205c-e) Plato aporetically concludes the dialogue in a skeptical circle (209e) in which all knowledge is apparently derived from non-knowledge, knowledge has no definite foundation, and all pretensions to knowledge results in ignorance.
In the Statesman, Plato introduces his penultimate method of division and combination, modeled on the art of weaving, for the purpose of transforming negative dialectic into positive dialectic by dividing and combining formal measures. Where in the Phaedo and the Republic, the formal measures had been conceived as transcendent a priori hypotheses of speculative ontology, in the Statesman these formal measures are instead postulated as the immanent a posteriori standards that are contributed from all of the crafts that are practiced within the political community: all of the other arts (288a-291c) are meant to contribute their positive formal standards as instruments for the weaving together of all forms by the kingly art of statesmanship. (287c-e) Yet Plato seems to recognize that this strategy of re-locating the formal standards to the immanent political community cannot answer the criticisms of the Theaetetus and the Parmenides when he cautions that “if we dismiss statecraft as unreal, we shall have blocked all means of approach to any subsequent study of the science of kingly rule.” (284b) Since the problems of anti-foundationalism and contradiction bedevil any possibility of constructing a speculative ontology, regardless of the location of the formal standards, Plato advises that a future science of dialectics (284c-d) can only be secured by a “prophylactic argument” against the “malady of doubt.” (282c-284a) The method of division and combination can, in this way, only construct a speculative ontology by dogmatically presupposing the nullity of skeptical doubt.
For further analysis of Plato see my lecture Plato, Logic, and Ontology