I thought context might help. Or not.
You may know Kant considers the “I” that thinks to be the ground of consciousness, just meant to provide that “….all my representations belong to me alone….”, such that there’s no chance for “…a varied and many-sided self as there are representations….”.
I bring this up because a distinction is required between the understanding that thinks, which is the same as taking for granted it can consider objects as things in themselves, and the “I” that thinks. Conceptions belong to understanding, they arise through spontaneity with respect to phenomena, and they also arise spontaneously even without the synthesis with phenomena, hence the intelligible object, and this is what it means for the understanding to think, the spontaneity of conceptions.
The “I”, on the other hand, that thinks, indicates only the synthesis of representations in consciousness, which means the conceptions represented are already given, which means understanding has already thought. This makes sense when we say, “I understand”, which makes explicit “I” as the transcendental ego representing the self, is not the same as the faculty of understanding, which is merely a logical functionary.
Here, too, when he says, “I can think whatever I please provided only that I do not contradict myself…”, it happens that when understanding considers the objects it thinks as intelligible existences, understanding is contradicting itself, by trying to apply a category, existence, to that which isn’t even available for it to be applied to, insofar as such availability requires sensibility, not mere thought.
Another way to think about it…..time. When we say, it just popped into my head, we’re talking about a sheer instant, that time when there was no considered object, and the next when there was. At that point of “there was”, there is no other cognitive constituency at work, there just hasn’t been any time for it.
Ok, so now let it be given the human cognitive system doesn’t stop working, there are no blank spots while conscious and otherwise naturally functional. So we got this time of a considered object, and if the system keeps on rolling along as it should, it will do what it supposed to do, which is to form a cognition of the thing it just took for granted as a considered object. So now let it also be given that the means for cognizing anything at all has certain requirements, and one of these requirements is synthesis of representations…..and we find there just aren’t any representations present in the system to be synthesized, at the time understanding merely considers an object in itself.