Like the other types of syllogism, a hypothetical syllogism may leave a premise unstated. Suppose you are indoors and cannot see the sky, but you infer that the sun isn't shining because there are no shadows on the ground outside. Your reasoning contains an assumed hypothetical premise:

(If the sun were shining, there would be shadows.) There are no shadows. Therefore, the sun is not shining.

In other cases, both premises are given, but the conclusion is left implicit; this is more common with hypothetical syllogisms than with other types of syllogisms.