In the philosophy of Parmenides the theme of ontology forms the prelude. Experience offered him nowhere a "Being" as he imagined it to himself, but from the fact that he could conceive of it he concluded that it must exist; a conclusion which rests upon the supposition that we have an organ of knowledge which reaches into the nature of things and is independent of experience. The material of our thinking according to Parmenides does not exist in perception at all but is brought in from somewhere else, from an extra-material world to which by thinking we have a direct access. Against all similar chains of reasoning Aristotle has already asserted that existence never belongs to the essence, never belongs to the nature of a thing. For that very reason from the idea of "Being"--of which the _essentia_ precisely is only the "Being"--cannot be inferred an _existentia_ of the "Being" at all. The logical content of that antithesis "Being" and "Not-Being" is perfectly nil, if the object lying at the bottom of it, if the precept cannot be given from which this antithesis has been deduced by abstraction; without this going back to the precept the antithesis is only a play with conceptions, through which indeed nothing is discerned. For the merely logical criterion of truth, as Kant teaches, namely the agreement of a discernment with the general and the formal laws of intellect and reason is, it is true, the _conditio sine qua non,_ consequently the negative condition of all truth; further however logic cannot go, and logic cannot discover by any touchstone the error which pertains not to the form but to the contents. As soon, however, as one seeks the content for the logical truth of the antithesis: "That which is, is; that which is not, is not," one will find indeed not a simple reality, which is fashioned rigidly according to that antithesis: about a tree I can say as well "it is" in comparison with all the other things, as well "it becomes" in comparison with itself at another moment of time as finally also "it is not," _e.g._," it is not yet tree," as long as I perhaps look at the shrub. Words are only symbols for the relations of things among themselves and to us, and nowhere touch absolute truth; and now to crown all, the word "Being" designates only the most general relation, which connects all things, and so does the word "Not-Being." If however the Existence of the things themselves be unprovable, then the relation of the things among themselves, the so-called "Being" and "Not-Being," will not bring us any nearer to the land of truth. By means of words and ideas we shall never get behind the wall of the relations, let us say into some fabulous primal cause of things, and even in the pure forms of the sensitive faculty and of the intellect, in space, time and causality we gain nothing, which might resemble a "_Veritas æterna?_" It is absolutely impossible for the subject to see and discern something