What are Paul's memories while he stands on sentry duty?

One of Paul's memories is of the cathedral:

The parachute-lights soar upwards--and I see a picture, a summer evening, I am in the cathedral cloister and look at the tall rose trees that bloom in the middle of the little cloister garden where the monks lie buried. Around the walls are the stone carvings of the Stations of the Cross. No one is there. A great quietness rules in this blossoming quadrangle, the sun lies warm on the heavy grey stones, I place my hand upon them and feel the warmth. At the right-hand corner the green
cathedral spire ascends into the pale blue sky of the evening. Between the glowing columns of the cloister is the cool darkness that only churches have, and I stand there and wonder whether, when I am twenty, I shall have experienced the bewildering emotions of love.

He also thinks about the poplar trees of his youth:

Between the meadows behind our town there stands a line of old poplars by a stream. They were visible from a great distance, and although they grew on one bank only, we called them the poplar avenue. Even as children we had a great love for them, they drew us vaguely thither, we played truant the whole day by them and listened to their rustling. We sat beneath them on the bank of the stream and let our feet hang in the bright, swift waters. The pure fragrance of the water and the melody of the wind in the poplars held our fancies. We loved them dearly, and the image of those days still makes my heart pause in its beating.

For Paul, the memories are calming.