What does John Donne mean with regard to the first sentence in "Meditation 17”?

With regard to the first sentence in "Meditation 17”, John Donne means that someone may have death at their door and not even really know it. The first line of Meditation 17 reads:

“Perchance he for whom this bell tolls may be so ill as that he knows not it tolls for him.”

Donne is saying that someone may be very ill - this could be physically and/or mentally, as Donne does not specify here. He leaves it up to the reader to decide in this very first line, although the typical reasoning here would probably be thinking of someone who has an injury or disease or serious sickness. Nonetheless, mental illness can lead to a person’s death as well.

Regardless, Donne is saying that often we think that serious sicknesses, diseases, and death are something that happens to others. Sometimes we think we are invincible and that death is far away from us. We carry on with our lives as if every day will be the same, whereby we will (without hindrances) pursue our dreams and passions, without anything stopping us or slowing down our plans.

The introduction to Donne’s Meditation 17 states:

“Now this bell tolling softly for another,

says to me, Thou must die.”

Here, it is saying that as death quietly comes to another person’s door, it is also on its way to someone else’s. We can be caught up in our lives and not know that we are courting death by what we are doing. Conversely, we may have a sickness, injury, or disease, and be using all our might to fight it (which is a good thing) but may not realize that even in our fight, death may still be near and be the victor in this struggle.

It doesn’t mean that a person should not do all he or she can to improve their health situation. It’s just that Donne is saying that despite human efforts we may not know that the bells tolling are for us and that we may be closer to death than we think. The opening line of Mediation 17, then, is a prod to human beings to live life to the fullest – one day at a time -  and to make the most of the time we’ve been given as our days are indeed numbered.