§ 158. It is scarcely necessary to point out that this insight, however negatively it be used, is a revelation of positive knowledge. Heraclitus and Parmenides claimed to know; Socrates disclaimed knowledge _for reasons_. Like all real criticism this is at once a confounding of error and a prophecy of truth. The truth so discovered is indeed not ordinary truth concerning historical or physical things, but not on that account less significant and necessary. This truth, it will also be admitted, is virtually rather than actually set forth by Socrates himself. He knew that life has some meaning which those who live with conviction desire at heart to realize, and that knowledge has principles with which those who speak with conviction intend to be consistent. There is, in short, a rational life and a rational discourse. Furthermore, a rational life will be a life wisely directed to the end of the good; and a rational discourse one constructed with reference to the real natures of things, and the necessities which flow from these natures. But Socrates did not conclusively define either the meaning of life or the form of perfect knowledge. He testified to the necessity of some such truths, and his testimony demonstrated both the blindness of his contemporaries and also his own deficiency. [Sidenote: The Historical Preparation for Plato.] § 159. The character and method of Socrates have their best foil in the sophists, but their bearing on the earlier philosophers is for our purposes even more instructive. Unlike Socrates these philosophers had not made a study of the task of the philosopher. They _were_ philosophers--"spectators of all time and all existence"; but they were precritical or dogmatic philosophers, to whom it had not occurred to define the requirements of philosophy. They knew no perfect knowledge other than their own actual knowledge. They defined being and interpreted life without reflecting upon the quality of the knowledge whose object is being, or the quality of insight that would indeed be practical wisdom. But when through Socrates the whole philosophical prospect is again revealed after the period of humanistic concentration, it is as an ideal whose possibilities, whose necessities, are conceived before they are realized. Socrates celebrates the rôle of the philosopher without assigning it to himself. The new philosophical object is the philosopher himself; and the new insight a knowledge of knowledge itself. These three types of intellectual procedure, dogmatic speculation concerning being, humanistic interest in life, and the self-criticism of thought, form the historical preparation for Plato, the philosopher who defined being as the ideal of thought, and upon this ground interpreted life.