1. A computer-implemented method for automatically detecting semantic errors in a natural language text document using ontology-based extraction rules, the method comprising: inputting to a computer the natural language text document; storing the ontology-based extraction rules in the computer, wherein the ontology-based extraction rules express formal logical relationships between classes and properties of an ontology for a predefined knowledge domain of relevance to the natural language text document, wherein the ontology-based extraction rules comprise extraction rules that express logically correct relationships between the classes and properties of the ontology, and wherein the ontology-based extraction rules comprise extraction rules that express logically incorrect relationships between the classes and properties of the ontology; matching by the computer sentences in the natural language text document with the ontology-based extraction rules to identify logically correct and incorrect statements in the natural language text document; outputting from the computer a list of the logically correct and incorrect statements in the natural language text document.
2. The method of claim 1 further comprising deriving a grade for the natural language text document based in part on the logically correct and incorrect statements in the natural language text document.