The Error
Leaving aside, for the moment, the idealistic position that tries to by-pass the bipolar nature of cognition as an action whose aim is to represent the features of an ontologically independent world, we note that the elementary constituents of such representations are expressed in statements which are qualified as true when the cognitive action is successful.
Unfortunately, cognition often fails to attain its goal and this is made evident by the common experience of mistakes and errors in our cognition. An error is expressed in a statement that does not represent the structure of the world to which it makes reference and, paradoxically, the existence and recognition of errors is one of the most convincing arguments for ontological realism. Indeed, how can I be sure that the world has a structure (i.e., that it is endowed with certain definite properties and relations)? The answer is that if it had no structure, whatever statement could be affirmed about it. Hence, if certain statements cannot be affirmed, because they are false, because they express an error, this means that they did not represent the structure of the world, hence the discovery of errors (and the possibility of correcting them) testifies of the ontological independence of the world from cognition.Truth is not a property of reality, but of representations, and consists precisely in the fact that a representation ‘corresponds’ to reality. But how can we have representations of what is not real? Moreover, how can we discriminate true from false representations? Parmenides had attributed to reason the exclusive capability of attaining truth by a clear understanding of the ultimate and simplest characteristic of reality, which is being, so that non-being, having no existence, cannot even be thought of and spoken of. All statements which common sense spontaneously accepts because they are supported by sense perceptions are—according to Parmenides— confined to the status of pure opinions that are almost certainly false since they would entail the existence of non-being (like the belief in the existence of change and of the multiplicity of beings).
Protagoras, however, taking seriously Parmenides’ thesis that non-being cannot be thought of and spoken of, maintained that all opinions are true, since they can be thought and discussed. A way out of this difficulty was offered by Plato by first clarifying that a negative statement is not the affirmation of a non-existent, but the affirmation of something “other” than what is being stated. In this way the existence of true opinions was duly recognized, and Parmenides’ distinction between opinion and truth was replaced by the difference between opinion and knowledge: knowledge was defined as true opinion supported by arguments providing its reasons (in modern terms we can say by a “justification”). This fundamental step amounted to linking knowledge with reality (due to the requirement of truth), and in addition with certainty (due to the requirement of rational justification).Already in antiquity, however, the availability of criteria or methods for securing both requirements was strongly debated. Sense perceptions were easily shown to be an insufficient ground, owing to their subjective privacy and also to well-known examples of sense illusions, while the possibility of finding intellectually evident first principles from which the justification of particular domains of knowledge would logically follow appeared as no less problematic. This is why skepticism surfaced from time to time during the history of Western thought.
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