The Scale of Likelihood

I thought it might be use­ful to define what I mean when I use words like ‘prob­able’ or ‘pos­sible’, so I’ve cre­ated my own scale to define how likely I think some­thing is to be sig­ni­fic­ant. It’s meant for whether or not an astro­nom­ical cor­rel­a­tion is mean­ing­ful to archae­olo­gical mater­ial, but it prob­ably has gen­eral applic­a­tions.

Example: The Sun rises over Stonehenge every day.
Something is cer­tain if, by neg­at­ing it, you effect­ively rule out any hope of hav­ing a mean­ing­ful dis­cus­sion. It is pos­sible that in the future data might prove things thought to be cer­tain are not, but here and now cer­tainty is the found­a­tion from where we start.

Example: Stonehenge has inten­tional astro­nom­ical align­ments in its archi­tec­ture.
For some­thing to be prob­able it has to have evid­ence in its favour AND be the only or far and away the most con­vin­cing explan­a­tion for a phe­nomenon. While a prob­able phe­nomenon is not cer­tain, reject­ing it would require an awful lot of extra explan­a­tion for events

Example: Stonehenge was built so that the entrance would face the rising Sun at the mid­sum­mer sol­stice.
For some­thing to be plaus­ible the concept has to have some evid­ence in its favour. There will more than one plaus­ible answer to a prob­lem. In this example the entrance does face the sun­rise. However the site could have been built so that people approach­ing from the avenue were facing the set­ting sun at the mid­winter sol­stice. I would argue the second is more likely, but the sum­mer option is not con­tra­dicted by the evid­ence, so the winter explan­a­tion I prefer can only be said to be a plaus­ible answer. Then there’s the other idea that both factors were delib­er­ately planned…

Example: Only the elites saw astro­nom­ical events from inside Stonehenge.
A pos­sible explan­a­tion is one that is based on reas­on­ing, but has no real evid­ence for it one way or another. A well-reasoned pos­sible explan­a­tion may be more con­vin­cing than a poorly-reasoned plaus­ible explanation.

Example: Astronomy played no part in the build­ing of Stonehenge
A prob­lem­atic explan­a­tion will have evid­ence against it which will require a lot of explan­a­tion to fix. There will be other more con­vin­cing explan­a­tions of the same phe­nomenon. I’d argue the example is prob­lem­atic because so many Stone Age and Bronze Age struc­tures do share sim­ilar orientations.

Example: The ali­ens that built Stonehenge came from Venus
An unlikely argu­ment is one that, if accep­ted, cre­ates no grounds for exclud­ing any other argu­ments at all.

The terms have no numeric val­ues because that would be a bit silly. They are also delib­er­ately bland. If someone prom­in­ent says some­thing stu­pid. I would like to say that “Prof X’s work is prob­lem­atic in places, and ulti­mately an unlikely the­ory”, rather than “Prof X’s work could be unpicked by a thir­teen year-old in five minutes.”

In terms of com­bin­ing the­or­ies, the hybrid will at most as likely as the weak­est com­pon­ent that makes up the hybrid. The com­bin­a­tion may also be less likely, but com­bin­ing two pos­sible the­or­ies will never make a the­ory plaus­ible. That’s for my use to remind me how likely some of my own work is.

I’m open to cri­ti­cisms of the defin­i­tions, the terms and even the name of the scale. I wanted to call it a Scale of Plausibility, but I can­not really if one of the terms defined is plaus­ible.


When he's not tired, fixing his car or caught in train delays, Alun Salt works part-time for the Annals of Botany weblog. His PhD was in ancient science at the University of Leicester, but he doesn't know Richard III.