Something to keep in mind at all stages of conlanging is the idea of mutual translatability:
All languages are capable of encoding the same information, and so whatever you say
in one language should theoretically be capable of being translated, however imprecisely,
into any other language.
What’s different is how the information is packaged and, crucially, what information
is deemed mandatory to convey.
One place this is especially useful to bear in mind is when discussing evidentiality.
In languages that exhibit evidentiality, every sentence comes with some form of marking that
indicates the source of the information being conveyed, whether it was observed directly,
reported from an outside source, or deduced from available evidence.
To a speaker of a language without evidentiality, this may seem like a really bizarre and oddly
specific thing to mark, but of course, all languages have means of stating the source
or veracity of the information expressed in any given statement.
In English, we have adverbs like ‘apparently’ and ‘seemingly’ which serve to express
that the information is second-hand or not directly understood, or we can use phrases
like ‘I saw that…’ or ‘It seems that…’ to identify the specific source of the information.
The difference is that in languages with evidentiality, this isn’t just an optional bit of extra
info, it’s an inseparable property of the grammar, usually being marked on the verb
in a similar way to tense, aspect, and mood.
According to linguist Alexandra Aikhenvald, evidentiality can be found in about one quarter
of the world’s languages, while more than half of all languages surveyed by the World
Atlas of Language Structures have some sort of grammaticalized evidential marking, so
it seems to be a surprisingly common feature.
However, different languages vary considerably in what categories of evidence they distinguish
between.
The most basic evidential systems simply mark whether or not the speaker has firsthand evidence
of what they’re saying.
In Turkish, for example, ‘Başkan geldi’ means ‘the president arrived’, but it
has the underlying implication that the speaker personally witnessed this event happening.
However, if we swap out the definite past tense suffix for the indirect past tense to
make, ‘Başkan gelmiş’, the meaning becomes ‘Apparently, the president arrived’, implying
that the speaker wasn’t there to see it, but knows about it by some other means that’s
left ambiguous.
More complex systems will make further distinctions based on the nature of the evidence, with
the most common subdivisions being reportative evidence or hearsay, meaning the information
was obtained from someone else, and inferential evidence, meaning the information can be deduced
from context.
A small number of languages go even further than this, dividing the ‘firsthand’ evidential
into different forms of sensory information, such as visual versus auditory information,
and a very small number of languages make even finer distinctions.
But how do these systems come about in the first place?
There are two main ways evidentiality can evolve.
The more common and more intuitive way is for evidential markers to derive from words
or phrases related to the source of evidence.
It’s very common for verbs of sensation, like “to see”, “to hear”, or “to
listen” to become grammaticalized evidentials.
In Tariana, the visual evidential is transparently related to the phrase “I see”, while the
non-visual evidential comes from the verb “to hear”.
Reportative evidentials almost always come from verbs of speech.
In Lezgian, the hearsay evidential evolved from a phrase that means “one says”.
Inferential evidentials may come from verbs like “to seem” or “to sense”, or from
adverbs like “probably”.
In this type of system the unmarked verb will most often imply direct or firsthand evidence,
and all other forms of evidence will receive overt marking.
Alternatively, the other method of evolving evidentiality is to derive the evidentials
from repurposed verb morphology, usually a tense, aspect, or mood that has to do with
result.
In the English phrase “Mary is gone”, the main verb is in the present tense, but
the verb “go” is rendered as a past participle, so the statement describes the present result
of a past action.
What this is essentially means is “Mary isn’t here now, because presumably, she
went out”, and therein, we have an inferential interpretation.
A similar process led to the evidential system in Bulgarian, in which the past aorist participle
is used to create four different evidential categories.
In Lithuanian, the active participle is used for reportative evidence, while the passive
participle communicates inferential evidence.
Although this type of system most often evolves from past or perfective morphology, evidentials
may also arise from or form an extension of a grammatical mood, particularly the subjunctive
or irrealis moods, and there are even some attested cases of evidentiality
evolving from future morphology.
One of the main differences between these two origins of evidentiality is that, due
to their etymology, evidentials derived from verb morphology will still have some lingering
tense, aspect, or mood information bundled into them.
In Turkish, for instance, the miş suffix conveys both indirect evidentiality and the
past tense in a single suffix, and thus occurs independently of the definite past suffix.
In fact, combining both suffixes results in a remote past interpretation.
This also means evidentials derived from verb morphology can often only occur in certain
tenses, aspects, and moods.
In contrast, lexically-derived evidentials are usually distinct from tense and aspect
morphology and aren’t as restricted in their distribution.
However, even then, evidentials are frequently used to convey numerous other subtleties of
meaning.
One question that you might be asking yourself is, what happens if the verb has a first person
subject?
Of course I was there to see what I myself did, so how can any non-firsthand evidentials
ever be used in the first person?
Well, let’s say you’ve fallen asleep on a train, and upon waking up, you see the train
has reached its destination.
In Turkish you might say “gelmisim”, “apparently, I’ve arrived,” with the evidential signalling
that up until this point, you weren’t aware of this.
Using 1st person evidentials like this to mark involuntary or unintentional actions
is a very common strategy cross-linguistically, including in complex systems with multiple
categories of evidence.
Along those same lines, evidentiality also frequently overlaps with mirativity, the marking
of information as surprising or counter to one’s expectations.
In Turkish, Yağmur yağıyormuş means “apparently it’s raining”, but it can also equally
mean “Oh, It’s raining!”, expressing surprise on behalf of the speaker.
Finally, another element that’s sometimes related to evidentiality is the dubitative
mood, which expresses information as doubtful or uncertain.
Sometimes the dubitative exists in complementary distribution to the evidentials, essentially
forming a separate category of evidence, or lack thereof, while in other
systems the dubitative can be expressed by means of the evidentials themselves.
In Turkish, a dubitative meaning can be accomplished by reduplicating the indirect past suffix,
so while gitmisler means “apparently they’ve gone”, gitmismisler means “It seems they’ve
gone, but I doubt it”.
Similarly in Bulgarian, the dubitative is formed by using the past aorist participle
on both the copula and the lexical verb.
These sorts of interactions with other modal elements should happen naturally as the system
evolves, so its worth having a play around with your evidential system and see what sort
of meanings you can extrapolate from mixing it with other morphology, keeping the etymology
of your evidentials in mind along the way.
So In summary, if you want to include evidentiality in a conlang, decide what categories of evidence
the language will distinguish between, choose the source of your evidentials, whether from
lexemes or verb morphology, work out how they interact with tense, aspect, and mood, an
decide what other uses or secondary meanings the evidentials might have.
All-in-all, evidentiality is a fun way of exploring various nuances of grammar and pragmatics,
and challenges you to a think about translation in a distinctly non-English way, and that
alone makes it a valuable asset to a conlang.