His statement, “by intention,” excludes the case where one changes from one obligatory prayer to another by beginning anew with the opening takbir—for the opening takbir is made verbally. Thus, in the example we mentioned: suppose he remembers that he prayed Zuhr while in a state of ritual impurity, so he leaves the Asr prayer and says the opening takbir for Zuhr. We say: his Asr prayer is invalidated because he has cut it off, while his Zuhr prayer is valid because he began it from the outset. This is why the author qualified it by saying, “by intention”; that is, not by beginning anew with the opening takbir.
His statement, “both are invalid,” is somewhat loose wording based on predominant usage. More accurately, it should be said: the first is invalidated, while the second does not commence, because invalidation applies only after something has commenced. Invalidation affects something valid and renders it invalid. But this is a matter of loose wording and predominant usage, just as Abu Bakr and Umar are called “the two Umars,” and the sun and moon “the two moons.” The difference of opinion in this matter is inconsequential.
It is understood from the author’s statement, “he changes from one obligatory prayer to another,” that if he changes from one voluntary prayer to another, neither is invalidated. This is the third category. However, this is not intended without qualification, for if he changes from one specified voluntary prayer to another specified voluntary prayer, the ruling is the same as when changing from one obligatory prayer to another. Thus, if he changes, for example, from the regular sunnah prayer after Isha to the Witr prayer, the regular sunnah prayer is specified and the Witr prayer is specified. The first is invalidated, and the second does not commence, because changing from one specified prayer to another invalidates the first and does not establish the second, whether it is an obligatory or voluntary prayer.
If he changes from a specified obligatory prayer, or from a specified voluntary prayer, to an unrestricted voluntary prayer, it is valid. This is the fourth category. In the case of an obligatory prayer, however, it is stipulated that sufficient time remains.
The reason is that a specified prayer includes two intentions: a general intention and a specific intention. If the specific intention is cancelled, the general intention remains.