I can tell that from the Hebrew various English translations understand something different has been put into the heart of man. What I am interested in then is does the LXX support one of the English translations? I looked up a Greek-English LXX interlinear but it was still not exactly clear to me what the meaning in Greek is.
Here is the Greek:
11 σὺν τὰ πάντα ἐποίησεν καλὰ ἐν καιρῷ αὐτοῦ καί γε σὺν τὸν αἰῶνα ἔδωκεν ἐν καρδίᾳ αὐτῶν, ὅπως μὴ εὕρῃ ὁ ἄνθρωπος τὸ ποίημα, ὃ ἐποίησεν ὁ θεός, ἀπʼ ἀρχῆς καὶ μέχρι τέλους. (The Lexham Greek-English Interlinear Septuagint)
Here are a few divergent English translations:
Ecclesiastes 3:11
ESV
He has made everything beautiful in its time. Also, he has put eternity into man's heart, yet so that he cannot find out what God has done from the beginning to the end.
NET Bible
God has made everything fit beautifully in its appropriate time, but he has also placed ignorance in the human heart so that people cannot discover what God has ordained, from the beginning to the end of their lives
KJV
He hath made every thing beautiful in his time: also he hath set the world in their heart, so that no man can find out the work that God maketh from the beginning to the end.
Does the LXX support 'eternity', 'world' or 'ignorance'? I understand the Greek word employed is 'age' so I sort of guess that somehow this indicates 'eternity' but really have no clue.