When material bread is eaten, it does not permanently take away our hunger, since it must be destroyed in order to build us up; and this is necessary if we are to be nourished. But spiritual bread, which gives life of itself, is never destroyed; consequently, a person who eats it once never hungers again....
One reason why temporal things do not take away our thirst permanently is that they are not consumed altogether, but only bit by bit, and with motion, so that there is always still more to be consumed. For this reason, just as there is enjoyment and satisfaction from what has been consumed, so there is a desire for what is still to come. Another reason is that they are destroyed; hence the recollection of them remains and generates a repeated longing for those things. Spiritual things, on the other hand, are taken all at once, and they are not destroyed, nor do they run out, and consequently the fullness they produce remains forever....
[St. Thomas Aquinas, Commentary on the Gospel of John, Chapters 6-12, Larcher & Weisheipl, trs., CUA Press (Washington DC: 2010), pp. 25-26.]
