Say you are stranded, and you do not have enough money for a taxi – not in cash and not in the bank. Someone offers you a ride and brings you to your destination. You cannot repay her, and she wouldn’t accept it anyway.
We do the math, and see that you are in debt to her: You have received more from her than you were able to give her. With the calculation complete, the mind is through. Yet a sense of gratitude remains.
What, then, feels gratitude? The heart.
Considering who God is, we see that He had no need ever to create us, and yet here we are. We owe our creation to Him.
Moreover: He sustains us at every moment. We persist because He is thinking of us, is breathing life into us, even as I write and you read these words. We are indebted to Him at every moment.
What is it, O man, that depends on your every breath, on your mere thinking of it? Who lives and who dies when you cease to think of them? Who is it that inhales when you exhale?
It’s not simply that we are short of funds; we could not, even in principle, repay God. He made us; the converse is impossible. He sustains us in existence; there is not one thing we could do to alter, add to, or threaten His existence.
The cynic fights this, complains perhaps that existence is not always such a blessing. One notices that he is still here, else we would not hear his complaint. To be alive is greater than death, and any appearance to the contrary is a matter of psychology. The opposite of existence is not negative, but no thing.
In other words, if we did not exist, we would be owed nothing, anyway. Somehow, we have something. It is the heart which allows us to feel gratitude for this.