7 All the rivers run into the sea; yet the sea is not full; unto the place from whence the rivers come, thither they return again.
[AD 373] Ephrem the Syrian on Ecclesiastes 1:7
The waters that the earth drank on the first day were not salty. Even if these waters were like the deep on the surface of the earth, they were not yet seas. For it was in the seas that these waters, which were not salty before being gathered together, became salty. When they were sent throughout the entire earth for the earth to drink they were sweet, but when they were gathered into seas on the third day, they became salty, lest they become stagnant due to their being gathered together, and so that they might receive the rivers that enter into them without increasing. For the quantity that a sea requires for nourishment is the measure of the rivers that flow down into it. Rivers flow down into seas lest the heat of the sun dry them up. The saltiness [of the seas] then swallows up [the rivers] lest they increase, rise up and cover the earth. Thus the rivers turn into nothing, as it were, because the saltiness of the sea swallows them up.

[AD 390] Gregory of Nazianzus on Ecclesiastes 1:7
As for the sea, if I had felt no wonder at its size, I should have felt it for its stillness, at the way it stands free within its proper limits. If its stillness had not moved my admiration, its size must have done. Since both aspects move me, I shall praise the power involved in both. What binding force brought the sea together? What causes it to swell yet stay in position, as if in awe of the land its neighbor? How can it take in all rivers and stay the same through sheer excess of quantity?—I know no other explanation. Why does so great an element have sand as its frontier? Can natural philosophers, with their futile cleverness, give any account of it, when they actually take the sea’s vast measurements with pint size pots of their own ideas? Or shall I give you the short answer from Scripture, the one more credible, more real, than their long arguments? “He made his command a boundary for the face of the waters.” This command is what binds the elemental water. What makes it carry the sailor in his little boat with a little wind—do you not find it a marvelous sight, does not your mind stand amazed at it?—to bind land and sea with business and commerce and unify for humanity such very different things? What springs do the first springs have? Look for them and see if you, a man, can discover or track one down. Who parted plains and hills with rivers and gave them free course? How do we get a miracle from opposites—from a sea that does not get out and rivers that do not stand still? What feeds the waters, what different kinds of food do they get? Some are nourished with rain, others drink with their roots—if I may use a rich metaphor to describe the richness of God.

[AD 395] Gregory of Nyssa on Ecclesiastes 1:7
You, whose period of struggle is short, do not become more lifeless than the earth, do not become more unthinking than the insensible, for you are endowed with thought and directed by reason toward life. Instead, as the apostle says, “Continue in the things you have learned and been convinced of,” in that steadfast and immoveable stability, since this also is one of the divine commands, that you “be steadfast and immoveable.” Let your sobriety abide unshaken, your faith firm, your love constant, your stability in every good thing unmoved, so that the earth in you may stand to eternity. But if any one, yearning for greater possessions and letting his desire become as boundless as a sea, has an insatiable greed for the streams of gain flowing in from every side, let him treat his disease by looking at the real sea. For … the sea does not exceed its boundary with the innumerable streams of water flowing into it but remains at the same volume, just as though it were receiving no new water from streams. In the same way human nature too, restricted by specific limits in the enjoyment of what comes to it, cannot enlarge its appetite to match the extent of its acquisitions; while the intake is endless, the capacity for enjoyment is kept within its set limit.
[AD 395] Gregory of Nyssa on Ecclesiastes 1:7
The sea is a receptacle for water which tends to flow everywhere; water never ceases to flow while the sea never grows larger. What is the goal of the water’s course which always fills the unquenchable sea? What is this influx of water which never fills the ever-constant sea? Ecclesiastes speaks like this that he may explain the insubstantiality of our frenzied pursuits which result from elements constituting man’s existence. If the sun’s course consists in this, it too has no limit; neither is there any succession between day and night, and the earth is condemned to remain ever unmoved. The rivers also labor in vain, for they are consumed by the insatiable sea which receives this constant inflow to no avail. If this is true, what about man who is subject to such elements? Why are we astonished at the rise and fall of a generation which follows a natural course because a generation of men always succeeds the one before it and so forth? What does Ecclesiastes cry out to the church? That you, oh man, who contemplate the universe, should understand your own nature. The wonders you behold in heaven or on earth, the sun or sea, should help explain your human nature. Sunrise and sunset resemble our human nature because they both have in common the one course [circle] of life. When we come into existence, we later return to our natural place. Once our life sets, our light passes under the earth which then lays hold of it.

[AD 395] Gregory of Nyssa on Ecclesiastes 1:7
The time of your struggle is short. Do not be more inanimate than the earth nor more foolish than [beasts] which lack feeling, for you are endowed with reason and the capacity to administer. Rather, as the Apostle says, “Continue in the things which you have learned and have been assured of” in steadfastness and constant stability. Since these words refer to the divine commands, “be steadfast and unmovable,” allow temperance to abide in your life along with firm faith, constant love and stability in every kind of beauty, that you may resemble the earth’s eternal stability. If anyone is greedy like the ocean with its expansive, boundedness desire which the inflow of waters cannot satisfy, let one be cured of his illness by looking upon the ocean. In this way he will not transgress his own bounds in the multitude of waters but will retain the same fullness without the addition of more water. In similar fashion pleasures arising from human nature with its present limit cannot expand its gluttonous appetite to keep pace with their great number; rather, the influx [of pleasures] does not cease even though our capacity for enjoyment is limited.

[AD 420] Jerome on Ecclesiastes 1:7
"All torrents flow into the sea but the sea is not filled. To the place from which the torrents come,
there they return to go." Some men believe that the fresh waters that
flow into the sea are either dried up by the burning sun above, or are feed for
the salt-thirsty sea. Here our Ecclesiastes,
the creator of the very waters, says that they return to the heads of the
springs by means of hidden passages, and always boil out from their deep
channels into their springs. The Hebrews
believed that the rivers or sea had more significance in the metaphor of man,
because they return to the earth, whence they originated. They are also called torrents not rivers
because they flow that much more forcefully, yet the earth however is not
filled with a great number of dead men.
More precisely if we go down to the deeper parts, the turbid waters
return to the sea where they used to remain.
And unless I am mistaken, apart from the additions to the text, nowhere
is the word 'torrent' found in a good context.
For "you will drink those with the torrent of your desire" [Psalms 35, 9.],
although "of desire" is written in an addition. On the contrary the Saviour was taken to the
brook Cedron [John 18,1.],
and Elisha at the time of persecution hid away in the brook of Chorat, which
even dried up. But the sea is not filled
up completely, in the same manner as the bloodthirsty daughters in Proverbs [Prov. 30, 15.].