Upwards
An overly literalistic reading of up & down plays right into a popular naturalistic argument--it contends that the Bible indicates that the heavens where God dwells are above us in the sky, but in modern times we have sent machines & people into the sky, and there's nothing up there like what the Bible describes. A rocket sent upward will go through various layers of the atmosphere and eventually enter the vacuum of space. Furthermore, because the earth is spherical, that portion of the cosmos that is above our head at this moment will, 12 hours later, be below our feet. Is the Bible wrong?
The flaw in this argument can be seen by means of a thought experiment. I'm writing this post in the USA; there are users on this site who are in Australia--we are almost exactly on opposite sides of the earth. If we both were to look up at the same time, we would be looking in opposite directions! Which of us is wrong? Neither.
People 2500 years ago did not have as detailed an understanding of the cosmos as we do today, and surely people 2500 years from now will be just as happy to ridicule the simplistic nature of the understanding we have at present. They may well have believed pi = 3 or that the spirits of the dead inhabit caverns deep in the earth, but that isn't relevant. If God could only communicate everything or nothing we would have to be content with nothing.
That God knows things beyond the confines of our understanding is expressed effectively in the poetry of Job:
17 Have the gates of death been opened unto thee? or hast thou seen
the doors of the shadow of death?
18 Hast thou perceived the breadth of the earth? declare if thou
knowest it all. (Job 38:17-18)
God recognizes our limited understanding and communicates at a level we can understand:
The entrance of thy words giveth light; it giveth understanding unto
the simple (Psalm 119:130)
(if we're willing to ask & seek--see Matt. 7:7)
If we are looking for the elevation of a specific realm, we will search the Biblical authors in vain. They didn't know. Plotting the Cartesian coordinates of Sheol was never the intention.
Does upwards describe God, or does God define what upwards is?
I propose the latter--God is upwards in that He exceeds the world around us in power, majesty, and glory. That is true regardless of the hemisphere in which this post is being read.
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A sincere question
That the "who knows" in Eccl. 3:21 is a genuine question, rather than a rhetorical exercise, is supported by comparison to Ecclesiastes' use of the same "who knows" expression in 2:19 & 6:12, where uncertainty is clearly expressed (see Pulpit commentary).
The writer is asking a question because he does not know.
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Don't count on skipping the layover in Sheol
Jacob was a righteous man, yet when he contemplated death we read:
and all his sons and all his daughters rise to comfort him, and he
refuseth to comfort himself, and saith, 'For -- I go down mourning
unto my son, to Sheol, and his father weepeth for him (Genesis 37:35
YLT)
Similar comments could be made with respect to Isaiah, as demonstrated in this post.
The hope for the righteous, then, appears not to be the opportunity to avoid Sheol (a place of diminished power & glory?), but that like the hiker on a mountain ridge, the journey may involve both descent & ascent, but the final destination is upwards.
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Conclusion
Is there a connection between Proverbs 15:24 and Ecclesiastes 3:21? Are both passages talking about the same thing?
There is a connection in that they both contemplate the afterlife, and the possibility of moving upwards (towards God); both appear more interested in the destination than the contours of the journey.
Can a person avoid going to Sheol at death (down) by living a righteous life (up)?
Doesn't sound promising, but the net trajectory can still be up. Because of the atonement of Christ, the Fall was a step downwards but forwards and ultimately towards God (see 1 Cor. 15:21,22,42; 1 John 3:2). Death can be too.