1. A method for automatically indexing a set of base documents, said method comprising: inputting, with a computer device, data defining all elements respectively contained within the base documents which meet predefined criteria and defining a corresponding meaning, the elements comprising an element to be checked and surrounding elements that surround the element to be checked; evaluating, with a trainable classifying apparatus in communication with the computer device which has been trained to recognize whether inputted data belongs to a corresponding classification category or not, the received data as to whether the element to be checked has the corresponding meaning, wherein the training has been performed based on a training sample generated for base documents in which the element to be checked is surrounded by the surrounding elements and has the corresponding meaning, the received data comprising data coding absolute or relative positions of the surrounding elements by corresponding text strings; and for those base documents where elements have been found to have the corresponding meaning, building, with the computer device, an index indexing the base documents, the index comprising the elements having the corresponding meaning with a corresponding reference to the document in which each element is contained.