Bring back definition of `UO:unit` to its original definition

[l-emele]

Description of the issue

We imported unit from Units Ontology but changed its definition that it can also encompass non-physical units like currency.

Ideas of solution

Briefly discussed at OEO dev meeting 82:

• Bring back original definition of UO:unit
• Use our adapted definition[^1] for UO:unit for a new class OEO:unit.
• Add new class OEO:physical unit and make it equivalent to UO:unit.
• Make OEO:currency a subclass of OEO:unit

[^1]: A unit of measurement is a standardised quantity of a quantifiable entity

Workflow checklist

• [ ] I discussed the issue with someone else than me before working on a solution
• [ ] I already read the latest version of the workflow for this repository
• [ ] The goal of this ontology is clear to me

I am aware that

• [ ] every entry in the ontology should have a definition
• [ ] classes should arise from concepts rather than from words

[l-emele]

So shall we do this as suggested? If so, I can implement.

[stap-m]

Yes, please go ahead. It is not a clean import, if we change the definition.

[l-emele]

I started implementing, see #1892.

While doing so, I re-read the original and adapted definitions of uo:unit:

• original: A unit of measurement is a standardized quantity of a physical quality.
• adapted: A unit of measurement is a standardised quantity of a quantifiable entity

According to both definitions a unit is a quantity. However, so far we had the axiom unit SubClassOf: 'generically dependent continuant'. I cannot remember why we did that, probably at that time we had not yet anything like quantity. Anyway, I suggest now to follow the definition and make oeo:unit a subclass of quantity.

[stap-m]

This is the OEO def of quantity: A quantity is a quality of a material entity where the magnitude of the quality can be quantified via a quantity value, which means that it can be expressed as a number and a unit. A unit is definitely not a quality of a material entity, and thus, not a oeo:quantity. I.e. mass is a quality, but kg is not. Since we import the class, we should connive that the def is not Aristotelian.

[l-emele]

At least, I would not say definitely not...

If you think of the definition of the kilogram [^1], it is surely a quality of a material entity. Same for the old definition of the metre based on the metre prototype ("Urmeter"). [^1]: A mass unit which is equal to the mass of the International Prototype Kilogram kept by the BIPM at Svres, France.

But it is beyond the scope of this issue. So I am fine to leave it as it is.

[stap-m]

To get a better overview over the relations between quantity, quantity value and unit in OEO, I made a diagram. In our case, unit is related to quantity value, and not directly to quantity. Therefore, I we should to refer to quantity value also in the definition. I made this proposal also in the review of PR #1892

[l-emele]

From oeo dev meeting 87:

• Compromise: Add an editor note on different views

[l-emele]

Proposal for the editor note:

The OEO developers have different views on the correct classification of a (physical) unit. On one hand, a (physical) unit can be seen as a standardized quantity and thus as a quantity itself. On the other hand, a (physical) unit can be seen as an entity of it own kind that together with the numerical value forms a quantity value. In the OEO, we have opted for the latter view and therefore implemented (physical) unit as generically dependent continuants.

@stap-m : What do you think about this proposal? Could you please try to improve the latter two sentences as these describe your view (and I never fully understand it)?

I would like to add this note to both unit and physical unit.

[stap-m]

The OEO developers have different views on the correct classification of a (physical) unit. On one hand, a (physical) unit can be seen as a standardized quantity and thus as a quantity itself. On the other hand, a (physical) unit can be seen as an entity of it own kind that together with the numerical value forms a quantity value. In the OEO, we have opted for the latter view and therefore implemented (physical) unit as generically dependent continuants.

Good. I made some further suggestions. What do you think? If you prefer your version of the second sentence, I am also fine with that.

The OEO developers have different views on the correct classification of a (physical) unit. On one hand, a (physical) unit is often referred to be "a standardized quantity", e.g. in the UO, and thus is interpreted as a quantity itself. On the other hand, a (physical) unit can be seen as a generically dependent reference measure that is not bound to a specific bearer. ~asthat an entity of it own kind that~ Together with the numerical value it can describe a quantity value. In the OEO, we have opted for the latter view and therefore implemented (physical) unit as generically dependent continuants.

[l-emele]

Thanks for your edits, now it is much better. I'll implement this now.