2: perchance I may think myself...toll for me, and I know not that.

While writing this meditation, Donne is deathly ill. Donne was deathly ill many times during his life. At this point in the meditation, Donne is saying that it is possible he is sicker than he thinks he is, therefore, the bell ringing for someone's death may actually be ringing for him and he does not know it. Those who were taking care of him might have caused it to be rung for him because they expect him to die soon.