Sensu is a Latin word meaning "in the sense of". It is used in a number of fields including biology, geology, linguistics, and law. Commonly it refers to how strictly or loosely an expression is used, but it also appears in expressions that indicate the convention or context of the usage.
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Sensu is a Latin word meaning "in the sense of". It is the ablative case of the noun sensus, here meaning "sense". It is used in a number of fields including biology, geology, linguistics, semiotics, and law, often accompanied by an adjective (in the same case). Three such phrases are sensu stricto – "in the strict sense", abbreviation s.s.;[1] sensu lato – "in the wide or broad sense", abbreviation s.l.;[2] and sensu amplo – "in a relaxed, generous (or 'ample') sense", a similar meaning to sensu lato.
When appropriate, comparative and superlative adjectives may also be used to convey the meaning of "more" or "most". Thus sensu stricto becomes sensu strictiore ("in the stricter sense" or "more strictly speaking") and sensu strictissimo ("in the strictest sense" or "most strictly speaking").
| Base phrase | Comparative | Superlative | Meanings |
|---|---|---|---|
| sensu stricto | sensu strictiore | sensu strictissimo | in the strict/stricter/most strict sense |
| sensu lato | sensu latiore | sensu latissimo | in the broad/broader/most broad sense |
| sensu amplo | sensu ampliore | sensu amplissimo | in a relaxed/more relaxed/most relaxed sense |
Current definitions of the plant kingdom (Plantae) offer a biological example of when such phrases might be used. One definition of Plantae is that it consists of all green plants, all red algae and all glaucophyte algae. A stricter definition excludes the red and glaucophyte algae; the group defined in this way could be called Plantae sensu stricto. An even stricter definition excludes green algae, leaving only land plants; the group defined in this way could be called Plantae sensu strictissimo.[3] Conversely, where convenient, some authors derive expressions such as "sensu non strictissimo", meaning "not in the narrowest possible sense".[4]
Another common usage is to follow sensu by a person's name. Thus "sensu Smith" means "in the sense intended by or used by Smith".[5][6]
A similar form is in use to indicate the sense of a particular context, such as "Nonmonophyletic groups are ... nonnatural (sensu cladistics) in that..."[7] or "...computation of a cladogram (sensu phenetics)..."[6]
A related convenient usage is in conjunction with a concept author citation ("sec. Smith", or "sensu Smith"), indicating that the intended meaning is the one defined by that author. (Here the meaning of "sec." is "secundum": "in accordance with".) Such an author citation is different from the (more common) citation of the nomenclatural author citation. The author citation refers only to the type of the name, the specimen or specimens that one refers to in deciding whether other specimens are members of that species or not. Given that an author (such as Linnaeus, for example) was the first to supply a definite type specimen and to describe it, it is to be hoped that his description would stand the tests of time and criticism, but even if it does not, then as far as practical the name that he had assigned will apply. It still will apply in preference to any subsequent names or descriptions that anyone proposes, whether his description was correct or not, and whether he had correctly identified its biological affinities or not. This does not always happen of course; all sorts of errors occur in practice. For example, a collector might scoop a netful of small fish and describe them as a new species; it then might turn out that he had failed to notice that there were several (possibly unrelated) species in the net. It then is not clear what he had named, so his name can hardly be taken seriously, either s.s. or s.l.
After a species has been established in this manner, specialist taxonomists may work on the subject and make certain types of changes in the light of new information. In modern practice it is greatly preferred that the collector of the specimens immediately passes them to specialists for naming; it is rarely possible for non-specialists to tell whether their specimens are of new species or not, and in modern times not many publications or their referees would accept an amateur description.
In any event, the person who finally classifies and describes a species has the task of taxonomic circumscription. Circumscription means in essence that anyone competent in the matter can tell which creatures are included in the species described, and which are excluded. It is in this process of species description that the question of the sense arises, because that is where the worker produces and argues his view of the proper circumscription. Equally, or perhaps even more strongly, the arguments for deciding questions concerning higher taxa such as families or orders, require very difficult circumscription, where changing the sense applied could totally upset an entire scheme of classification, either constructively or disastrously.
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):
or sensu lato (or latiore):
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:
or the criterion could be very casual, even vague:
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.
Sensu is used in the taxonomy of living creatures to specify which circumscription of a given taxon is meant, where more than one circumscription can be defined.
Examples: