Reasoning from a sign to an underlying cause can be merely probable, or sufficiently conclusive under justified conditions. The label innī identifies the role of the middle term; it does not automatically condemn the inference as a guess.
An unlit lamp ordinarily permits several explanations. But in a specified closed system, if alternatives have been ruled out and the diagnostic relation established, the inference may become much stronger.
Do not confuse the kind of middle-term explanation with the certainty or probability of the premises.