There is a duality of 'I am' and 'it is' as primary expressions. Who is it that experiences? 'I'. What is? 'This'. The terms respond to questions, but only certain questions are able to bring attention up short in this way. How, why, when call for explanations, and while who and what can do so as well they also can require to be be answered by ostensions, and it is in this sense that they do their work of paralysing the stream of thought. 'I am' is or points towards transcendental subjectivity, while 'it is' points to transcendental consciousness. You can ask the what question of the 'I am', this is piling the what on top of the who: what is the phenomenon on which the meaning of the who question depends, or you can reverse it and ask who it is that asks what. Every 'this' is a this for a who, but the who is itself a sort of this, and so on. These questions attempt to turn the arrow of attention back onto its source. This is impossible, but there is nothing to stop you from making the attempt - the questions can be framed, as questions they are more or less coherent with our general range of meanings, even if reflection would rule them out. The point is not to find an answer to them however, but to use them to force a certain kind of gap in the meaning making process, to allow us to glimpse how thickly we are mired in it, how we are it.