Textual criticism is the technique or science of restoring manuscripts that are copies to as near as possible to their original form. Texts in this connection are defined as writings, inscribed or printed on paper, parchment, papyrus, or similar materials. The original text is most often referred to as the Autograph and the object is to get back as near as possible to this autograph. The aim and the method are the same whether it is the Bible or one of the classics by Homer, Pliny, Josphesus, or even Shakespeare.
The method simply put is to collect as much data as possible and compare any differences between the different texts. Any differnces are know as variance and the majority of variants are not important things such as spelling. It is common for the spelling of peoples names and places to have many different ways of being spelt (as still happens today). This accounts for a large majority of variance that are found in documents. People may feel that to have a greater amount of variance is a bad thing but this is not necessary the case because is more an issue of how many copies do you have. If you only have one copy you will have zero variance. With two copies you could have dozens depending on the copyist and length of the text. With the New Testament for example, there is such a wealth of copies (5800 in Greek alone) that we find there are more variants, than actual words in the New Testament. This may seem strange at first but when you considor that one scribe wrote the name John in six different ways in one document, you can see how this can happen.
Having a large number of Manuscripts is not necessary the best guide to geting back to the original. Having copies near to the time of the original is the best guide to getting back to the original. Of course there are other factors that affect the quality of a copy but you can find out about reading the books that available on this page as free pdfs.
Dr Daniel Wallace, Executive Director of CSNTM & Senior Research Professor of NT Studies at Dallas Theological Seminary, commented that scholars are confident for 99% of what goes into the New Testament. He also comments that of the 1% that they are not sure of, none of them challenge any of the doctrines of the Christian faith.
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Bruce Manning Metzger (February 9, 1914 – February 13, 2007) was an American biblical scholar, Bible translator and textual critic, a longtime professor at Princeton Theological Seminary and Bible editor of the American Bible Society and United Bible Societies. He was a scholar of Greek, and New Testament textual criticism, and wrote prolifically on these subjects. Metzger was one of the most influential New Testament scholars of the 20th century.
From Wikipedia
Frederick Fyvie Bruce FBA (12 October 1910 – 11 September 1990) was a Scottish evangelical scholar, author and educator who was Rylands Professor of Biblical Criticism and Exegesis at the University of Manchester from 1959 until 1978 and one of the most influential evangelical scholars of the second half of the twentieth century.
From Wikipedia.