Eternity in the heart
What the Preacher means by the eternity (or world) set in the human heart.
Ecclesiastes 3:9-15
9What profit has he who works in that in which he labors?
10I have seen the burden which God has given to the sons of men to be afflicted with.
11He has made everything beautiful in its time. He has also set eternity in their hearts, yet so that man can’t find out the work that God has done from the beginning even to the end.
12I know that there is nothing better for them than to rejoice, and to do good as long as they live.
13Also that every man should eat and drink, and enjoy good in all his labor, is the gift of God.
14I know that whatever God does, it shall be forever. Nothing can be added to it, nor anything taken from it; and God has done it, that men should fear before him.
15That which is has been long ago, and that which is to be has been long ago. God seeks again that which is passed away.
What each tradition says
Rabbinic / Midrashic
Rashi reads the "world" set in the human heart as a sense of the external world. The Holy One also instilled the wisdom of the world into human hearts, but not all into each person—a little to one and a little to another—so that man should not fully grasp the workings of the Holy One.
Rashi (The Metsudah Five Megillot, Lakewood, N.J., 2001), Rashi on Ecclesiastes (The Metsudah Five Megillot) - Sefaria — Comment on Ecclesiastes 3:11 (He has set the world in their heart)
Patristic
No sourced position found. Checked Gregory Thaumaturgus, A Metaphrase of Ecclesiastes (ANF 6, newadvent.org/fathers/0602.htm): chapter 3 paraphrases the 'times' material but never renders the 'world in their heart' clause of iii. 11; no other public-domain patristic exposition of Ecclesiastes 3 was found.
Catholic
Haydock notes that "consideration" is literally "dispute," while the Hebrew and Septuagint read "heart." Pagnin renders this as "He has implanted the desire of immortality in their hearts." Calmet says that if we could discover each thing's properties we would be in raptures, but Menochius says our inability to do so increases vexation.
George Haydock (with Calmet, Menochius), George Haydock's Catholic Bible Commentary — Comment on Ecclesiastes 3:11
Orthodox
No sourced position found. Checked NPNF ser. 2 vol. 9 (John of Damascus, ccel.org cache): 'Ecclesiastes' occurs only in the volume's book-lists and scripture index; Philaret's Longer Catechism (pravoslavieto.com): no commentary on the book; Gregory of Nyssa's eight homilies on Ecclesiastes have no public-domain English translation (Hall's edition is 1993).
Reformed / Protestant
Matthew Henry cites Bishop Reynolds, who holds that having the world so much in our hearts is a reason we do not know so much of God's works as we might. Being taken up with thoughts and cares of worldly things, and exercised in travail concerning them, leaves neither time nor spirit to eye God's hand in them.
Matthew Henry, An Exposition of the Old and New Testament, Vol. III (Ecclesiastes) — Comment on Eccl. iii. 11 (He has set the world in their hearts)
Evangelical
Wesley says God has set the world in human hearts, making people capable of observing all his dispensations in the world. Yet this has a limit: some of God's more mysterious works no one can fully understand, because they cannot be searched out from beginning to end.
John Wesley, Wesley's Notes on the Bible, Ecclesiastes (Wesley Center Online) — Chapter III, note on v. 11 (He hath set the world)