Shell Shock or Cowardice? - The case of Harry Farr

Private Harry Farr was a British soldier executed for alleged cowardice during the Battle of the Somme. His fate was particularly tragic because he had a history of "shell shock".  Last year, after many years, Harry and all the others executed for military offences during the First World War were finally granted a posthumous pardon. But what exactly had happened to Harry on that fateful day when he refused to go into the trenches?  The vast majority of those sentenced to death by British Court Martials were reprieved - why wasn't Harry?  What did shell shock really mean in 1916?  Finally, is it acceptable to judge history by our own contemporary standards?