The song was written for Syd Barret. However, we all relate since we’ve all had someone special in our life that chose a different path. This is the way I would interpret it.
So, so you think you can tell Heaven from Hell, blue skies from pain.
(Just because Syd chose a different path and because he thinks he knows what he’s doing doesn’t make it true or right… on the contrary the author of the song is telling him that he is wrong)
Can you tell a green field from a cold steel rail? A smile from a veil?
(When one is addicted to something they think they know better than everyone else. The author is trying to make Syd understand that what he’s doing is wrong, thus wants him to think twice about his life)
Do you think you can tell?
(And asks again… it’s like: are you sure you want to be doing this?)
And did they get you trade your heroes for ghosts?
(They: he’s referring to Syd’s inner voices that keep him straying… so, everything he believed earlier doesn’t stand anymore? (Syd’s beliefs that is))
Hot ashes for trees? Hot air for a cold breeze?
(Past experiences (good times) for new greater ones. However, the author says here that these will not be the same, what’s gone is gone)
Cold comfort for change? And did you exchange
(Again, points out to Syd that he wrongly believes that change is better than what he’s used to)
a walk on part in the war for a lead role in a cage?
(Asking if he’s exchanged something he could’ve got himself out of for a lifetime trap. In other words if it’s too late to turn back, and if the drugs have completely taken over his life)
How I wish, how I wish you were here.
(The author here gets nostalgic and wishes that Syd was there with him like the good old times)
We’re just two lost souls swimming in a fish bowl, year after year, running over the same old ground. What have we found?
(Two soul mates, the author and Syd, who went different ways. I believe that there are two fish bowls, each one trapped in their own, realising that they’re just going into circles)
The same old fears, wish you were here.
(All those years of experience and nothing is gained, back to square one… he wishes Syd was there with him to share that moment)