Since 1888 those that have been named as possibly being Jack the Ripper must number in their hundreds and that's just the ones we know about! How do you decide on a suspect? The answer to that depend on the criteria that you choose.
Does there have to be incontrovertable proof that the suspect really existed? If the answer is yes then Dr Stanley is not an option as it has never been conclusively proved that he did exist. "I plead not guilty of the crimes of which I am accused your honour on the grounds that I am a fictional creation." does appear a rather good defence! Until recently Michael Ostrog was also in this category but research in recent years has proven his existence.
Does the suspect have to have an East End connection? If so then Joseph Barnett and Donston are possibilities. How much of a connection is needed? Does having chambers at King's Bench Walk and a mother in an asylum at Clapton (Druitt) count or visiting at intervals from Liverpool (Maybrick)?
Is proof that the suspect knew at least one of the victims necessary? Again Joseph Barnett fits the bill as does George Hutchinson and Michael Kidney but so too does Dr Barnardo who claimed to have met Liz Stride only days before her murder and he spent a lot of time in the East End!
How many victims were there? If it's more than five and Alice McKenzie and Francis Coles are included then Druitt is ruled out as he died in December 1888 as is Tumblety who fled the country. The Maybrick diary only mentions the five usual victims so if there were more, or indeed less, he is also removed from the frame.
What about the various supposed Ripper writings? If the dear boss letter is genuine does it point towards an American? If the letter sent along with a kidney to George Lusk was really from the killer then it suggests a certain amount of theatricality and could have come from a poet ( J K stephen) or a bad actor (Druitt, whose Sir Toby Belch at school was better imagined than described).The Goulston Street graffito if written by the Ripper could point towards somebody who carried chalk in their pockets like a schoolteacher (Druitt). If the message is supposed to accuse the Jews then does that mean the killer was not a Jew or that he was and wanted to get other Jews into trouble? In this case the spelling "Juwes" suggests an uneducated person and not a schoolteacher, Cambridge scholar etc. Is Juwes a masonic term and if so does this mean a masonic conspiracy?
Did the killer show any medical skills? If so then Donston, Tumblety, Gull, Cream are some obvious choices. Would Druitt, who was not a doctor but came from a family of doctors, have enough skill? Could a butcher or Shochet have done it or was there no skill involved at all?
Should the suspects be limited to those named in police sources, official or otherwise? This would make the main candidates Druitt, Ostrog, Kosminski (named by Macnaghten), Tumblety (named by Littlechild), Chapman (named by Abberline) Donston (name in police files) plus a number of people like Barnett, Hutchinson and Kidney who were mentioned in relation to the case. Did the police know who the killer was? If they did and did not reveal the culprate then that suggests a conspiracy which in turn suggests the killer was important. Freemasons perhaps or a prince of the realm?
Should known murderers head the suspects list such as Chapman, Cream, Deeming, James Kelly and or should they be dismissed because their modus operandi was different from the Ripper's?
The above shows some of the many questions that must be answered before a supect can be settled on. Even when one is chosen that is not the end of the difficulties. The theory must be made to fit the facts and not the other way round. "Facts" which strenghthen the case of a suspect should not be blindly accepted and those that weaken the case should not be blindly ignored. The one certainty in the mystery of Jack the Ripper is that one can never be absolutely certain who was guilty. As Donald Rumbelow wrote "...the answer must always be 'Perhaps'. It can only remain conjecture."
Sources
Begg, Fido & Skinner The Jack the Ripper A-Z
Donald Rumbelow The Complete Jack the Ripper
