@marciag,
Kant is saying, as I read him, that if there is to be such a thing as knowledge of external objects, then there must be "self-consciousness"(memory, for example, that allows comparison) of the person experiencing them. He argues that the this person, or subject, experiences them according to certain common categories.
For consciousness, there is not a "that object" but just-that- particular- object- and-not- another; we do not immediately intuit an
object, but given sense-stuff/data:
"Now all experience does indeed contain, in addition to the intuition of the senses through which something is given, a concept of an object as being thereby given, that is to say, as appearing." (A93/B126)
Kant maintains, then that to formulate an OBJECT, a this THING, it must be conditioned by both the Transcendental Aesthetic (space and time) as well as the Transcendental Analytic (the twelve Categories of the Understanding.
I have a bunch of sense-data intruding on my self; it is a rock (of such a shape and place, heavy, there is only one of them, it is now just in front of me, and so on). It is what it is not because of the raw sense-data, but because it is conditioned by a unified Self, and these conditions are the same for everybody.
This is a better explanation than mine:
Kant: Transcendental Deduction