The principle of parsimony in science

The principle of parsimony (also known as Ockham’s Razor) is a fundamental aspect of science, yet is often overlooked. I was only formally introduced to it during my PhD which in hindsight looks like an incredible oversight. It was not that I was not aware of it, or was not putting it into practice (hey, I was already a cladist) but it had never been spelt out in absolute terms and what that means for science. Frankly how this is not taught at say age 12 or 13 is beyond me.

In short the principle of parsimony is one of simplicity – we should not go looking for more complex explanations when a simple one will do. What that really means in practice is we should go with the weight of the evidence available to us. This will probably seem very obvious, but in practice it is essential that we have a philosophically justified method of choosing between explanations of our data. After all when there is good evidence to support one idea and only slightly less good evidence to support another – can you really chose between them? Well, yes. You *MUST* take number 1. Philosophically, if you can choose a slightly less good answer then why stop there? Why not take any old explanation you like? As a scientist I have no problem with someone saying ‘you know, I don’t *like* that explanation, I think the other is more likely’, provided they do not accept it. Make your reservations known! Hey, if you are right and later on the evidence supports it, you look brilliant, but do not work under the assumption that your hunch is right, or more importantly that the other is wrong. That is the point of science, and of parsimony.

Going back to the stuff on pterosaur origins, (sorry, it’s them again) many researchers think that ultimately pterosaurs will turn out to have an archosauromorph ancestry, not a dinosauromorph one. They might be right, and it is worth considering as the evidence only just supports the latter conclusion. But it does support that conclusion, and we must assume that it is correct until the evidence points to another explanation even if we are not happy with it. I also wanted to deal with another aspect of parsimony here that relates to the previous post on pterosaur cryptozoology. Here as everywhere else in science parsimony must rule – something many of the cranks seem to forget, ignore or more likely, simply do not know about. Here it works in its purest form in that ‘the simplest explanation of the data must be correct’. In other words, the simplest (and therefore most parsimonious explanation) of a pterosaur sighting is not that it was a pterosaur (thus defying a healthy amount of the fossil record, absence of corroborating data like footprints, photographs) but that it was an error or over interpretation of the events. They saw something else and thought it was a pterosaur. In fairness to many of the ‘sighters’ things can be very confusing in the field. I have a very extensive background in biology with all kinds of exotic animals (I used to work in two zoos) and have travelled extensively looking at wildlife in the field (both professionally and for fun). I know for a given location what I expect to see, what I should not see, and what it might look like, and what else it might be and I have had a lot of practice at making those judgements. And I have still got it badly wrong – often. Part of this is how things appear in reality as opposed to how you see them in your head – a condor is easy to tell from a turkey vulture in a cage, or from a photo – it is very hard when 200 metres up in the sky, backlit by the sun and with nothing to scale it against. Also, things might not be as you expect – tail feathers might be broken and oddly arranged, it might have juvenile plumage, or have an unusual colour, or be mangy with most of the feathers missing, of have large leaves stuck to their feet. All of these have happened to me and as a result I made a poor identification that was resolved when the thing got closer or I had a second opinion. How many errors I made because I had no binoculars or any help I dread to think. Therefore one cannot blame every non-biologist who spots something moving fast above his head in an unfamiliar environment and does not get the scale right, or even realises that it was just an unusual bird and not a pterosaur.

However, much blame must also be attached to the crypto-crowd who will take everything at face value. As anyone who knows about police work will tell you, there is nothing so unreliable as eye-witness testimony. Add to that the problems described above and even before we introduce pterosaurs to the problem, when an untrained guy says that they correctly identified an unknown animal first time in unusual conditions I would be really sceptical. When they say it was a pterosaur, my skeptic-ometer goes ballistic, yet time and time again, a single eye-witness who is not a biologist (and can barely tell a dog from a cat) can apparently correctly identify a pterosaur (something they have never seen before) from every possible bird and bat species in the area, with all of the variations I mention, at a distance, in one go, and people believe him. When it comes to this kind of thing, there is only one option, and that is parsimony. The guy made a mistake. He misidentified some bird, or bat, or leaf in the wind, or plane or well, something. Not knowing what it was (and some birds and bats look very, very odd) he picked what was to his mind a rational explanation – a pterosaur, and likely his imagination filled in a few details (teeth and a tail-vane) after the event.

In short, just because someone said they saw a pterosaur does not mean they did. Maybe it happened, but let’s face it, if this is a good spot for pterosaurs, sooner or later someone will see one properly, or find a dead one, or take a photo, or find some footprints, or an egg. Why get excited now? If it is genuine, the evidence will follow sooner or later. For some strange reason though, despite thousands of sightings of some ‘common’ cryptids in even very well populated areas (like the Loch Ness Monster, or Thunderbirds) absolutely no evidence has turned up. There is a good reason for that, and parsimony tells us what it is.

This is a revised version of a Mk.1 post, to see the original with comments etc. go here.

3 Responses to “The principle of parsimony in science”


  1. 1 Hannah 06/10/2013 at 8:51 pm

    Thank you I found this extremely helpful

  2. 2 Saad Ahmed 10/08/2014 at 6:21 am

    If i want to achieve success or heaven …. following the principle of parsimony, what can be a single (parsimonical) cause to the desired effect (success or heaven) ?


  1. 1 Keep It Simple,Stupid (KISS/Occam’s Razor) | United for Ayla Trackback on 09/11/2013 at 5:36 pm

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