The deductive mood is an epistemic

Epistemic modality

Epistemic modality is a sub-type of linguistic modality that deals with a speaker's evaluation/judgment of, degree of confidence in, or belief of the knowledge upon which a proposition is based. In other words, epistemic modality refers to the way speakers communicate their doubts, certainties, and...

 grammatical mood

Grammatical mood

In linguistics, grammatical mood is a grammatical feature of verbs, used to signal modality. That is, it is the use of verbal inflections that allow speakers to express their attitude toward what they are saying...

 that indicates that the truth of the statement was deduced from other information, rather than being directly known. In English, deductive mood is often indicated by the word must, which is also used for many other purposes. By contrast, some other languages have special words or verb affixes to indicate deductive mood specifically.

An example in English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

:

There's gas in the house! Someone must have left the stove on!