The first undeniable reality is that every living thing dies, and the second undeniable reality is that we suffer throughout our lives because we don't understand death. The truth derived from these two points is the importance of clarifying the matter of birth and death. The third undeniable reality is that all of the thoughts and feelings that arise in my head simply arise haphazardly, by chance. And the conclusion we can derive from that is not to hold on to all that comes up in our head. That is what we are doing when we sit zazen.