Very good question, Matthew. Let me try out an idea on you. Your P101 is
actually a role played by a pump, rather than a pump itself. Think of it as
being like Hamlet, as played by Lawrence Olivier (P101 as played by S3556). You
can change actors, and Hamlet is still Hamlet - same role - and while Olivier
is playing the role, he *is* Hamlet, at least in a sense. But this second "is"
cannot be identity, since you can kick the actor, but you can't kick a role. (01)
Both a pump and a pump-role are spatiotemporal entities, but they have
different identity conditions. The identity of a pump, like any other physical
object, is determined by the disposition of pieces of material stuff (metal,
plastic, rubber), but the identity of the role is determined by its interfaces
to the rest of the system (being connected to this pipe in this place and
operated by this controller, etc..) (02)
You can identify a pump-phase (temporal slice) with a pump-role-phase, but you
must not identify the actual individuals, so its safer to actually have a
relation of 'functioning as' of the like to attach a role-playing thing to its
role. Or, you can treat the role as a time-dependent property of the physical
thing, but you will probably need a CL-style ability to have properties of
properties if you go that (elegant) route. (03)
Make sense? (04)
Pat (05)
On Jan 29, 2012, at 3:48 AM, Matthew West wrote: (06)
> Dear Colleagues,
>
> Last Thursday I complained that most ontologies do not give adequate
> treatment to what I call system components, and if ontology is going to gain
> traction within the systems world, it needs to get a better understanding of
> this central idea in systems engineering.
>
> I illustrated the issue by telling the (simplified) life story of a system
> component: the pump, P101, at the bottom of a distillation column. Here is
> its story.
>
> The designer creates a drawing of the distillation column including at the
> bottom of the column a pump to pump away the column bottoms. He labels it
> P101, decides that one pump will be sufficient, and gives the specification
> for the pump in terms of Net Positive Suction Head, differential head, flow
> rate, materials of construction, and many other things.
>
> The construction engineer picks up the drawing and specification and notices
> he has to install a pump as P101. Fortunately, he has a pump in stock from a
> previous project, that has been in stores unused for 5 years which exactly
> meets the specification. On it is stamped Serial No S3556.
>
> The designer and the Operator comes to see the pump be installed, and once
> the connections are made, he gives the pump a friendly kick and says to the
> construction engineer "It's good to see P101 realized at last". The
> construction engineer says in return "Yes, and it's good to get S3556 off my
> hands at last." He turns to the operator and says "Why don't we change your
> drawings to show S3556 instead of P101?" The operator says "No, don't do
> that, it's a replaceable part, and one day another pump will be put there,
> and I don't want to have to change all the drawings and other documentation
> that refers to P101 each time it is replaced, as far as I am concerned it's
> the same pump whatever is installed there."
>
> Some time later the pump breaks down and needs to be taken back to the
> workshop. The maintenance engineer says to the operator "Hi, can I take
> S3556 installed as P101 back to the workshop?" The operator replies "Sure,
> but what am I supposed to do without my P101? If it does not exist I cannot
> operate my distillation column." The maintenance engineer responds, "I
> understand. We have another pump S4567, that meets the same specification as
> P101. We'll replace S3556 with it and you will only be without P101 for a
> few hours. I don't understand how you can continue to call it P101 though
> when all the parts have changed at once." The operator replies "I don't care
> about that. What I care about is what is connected in my system to pump the
> liquid from the bottom of the column. As long as it does that, it is P101 to
> me."
>
> Later the distillation column is demolished. The operator says, "A sad end,
> I was very fond of P101, but it is no more." The demolition engineer says,
> "Yes indeed. Fortunately, we can take S4567 and use it on another plant."
>
> It's probably worth summarising the key characteristics of a system
> component:
> - It comes into existence the first time it is installed.
> - It is identical to the equipment items installed, whilst they are
> installed (but not before or after).
> - It can survive complete replacement of all its parts at once.
> - It can survive periods of non-existence.
> - It ceases to exist when the system it is a component of ceases to exist.
>
> This is clearly rather different from the life of ordinary physical objects.
> However, relatively few ontologies recognise that such things exist. Many
> try to fob system components off as being classes, or abstract individuals,
> though these clearly do not have the required characteristics.
>
> Ontologists need to step up to the mark here and provide proper recognition
> for system components.
>
> Regards
>
> Matthew West
> Information Junction
> Tel: +44 1489 880185
> Mobile: +44 750 3385279
> Skype: dr.matthew.west
> matthew.west@xxxxxxxxxxxxxxxxxxxxxxxxx
> http://www.informationjunction.co.uk/
> http://www.matthew-west.org.uk/
>
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