Sensu - Circumscription


Readers unfamiliar with technical aspects of taxonomy might find it helpful first to think of everyday examples of the principles. When dealing with groups and parts of groups (subgroups) of different types of things, taxonomists sometimes wish to speak of the full set under consideration, and sometimes just a subset, but almost always want to refer to some particular part, to the exclusion of other elements; in issuing an instruction to poll the opinions of twenty-one members of a village community, a competent pollster would not accept the reactions of two heads of households, three infants, four dogs, five cats, six rats, and a tramcar. That would be taking sensu lato beyond good sense.

Instead the instruction should specify which sense should apply, such as sensu stricto (or strictiore):

* "...all the heads of households on the north side of the stream," or "...all the children in hospital with mumps", or "...the men the district attorney questioned this morning," or "Zachiariah Quenton Horton of 221b Baker Street".

or sensu lato (or latiore):

* "... five of the school football team", or "the first few friendly-looking people you find in the street," or "...some of the people in the district."

The important thing is that in each example the instruction circumscribed the appropriate subjects; that means that the interviewer could tell which people were wanted and correspondingly, which were to be left out.

The circumscription could be in terms of very specific criteria:

(...of all the possible people, only those the DA questioned, and of those, only the adult males, or one specific person only)

or the criterion could be very casual, even vague:

( many as you like of the people that looked friendly to you in the street, even if it turns out that the appearance was misleading.)

However simple that may sound, it is fundamental both in formal science and in everyday affairs. Circumscription amounts to the basis for telling things apart, which in turn is the rational basis for all diagnoses, formal or informal.

In biological taxonomy, as the next section describes, the same principles apply, but they deal in various ways with circumscribing living things according to any relevant criterion. In modern biology the criterion usually has something to do with which creature descended from which kind of ancestor, in which ways it changed in the process, and by how much. However, in more general taxonomies, although the principles of circumscription are fundamentally similar, the criteria could be largely different in type as well as in detail.

In short, in every discipline the sense of circumscription in taxonomy must reflect the nature of the subject matter.

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