If someone grew up in a cubicle as Plato once suggested
They would only know the cubicle and not the world outside it
Referring to Plato’s theory called the allegory of the cave. He gave an example of 10 to 12 prisoners locked inside a cave since birth, and goes into a little more detail but this is all you need to know to understand this line (The rest of the theory will inevitably be explained anyway). What Eyedea is saying here is that If you and the people around you have been doing something/living the same way since birth, you have nothing to compare it to.
Kind of going off what you and Lobes have said, though I may take a different direction, I don’t think this is necessarily a reference to the allegory of the cave, though I’m not sure because I haven’t read a whole lot of Plato. However, I think the message he is trying to get across, regardless of what metaphor or figure of speech is being employed here, is that there is no way to step outside of the human brain and experience things any differently (this is the cubicle in this analogy). The cubicle being the human brain, we are not able to step outside of it and view reality for either what it truly is or from a different perspective (on a side note, Eyedea mentions in some of his other songs the “ultimate reality”, i.e. the truest, most “real” form of reality, hence why I think this is in reference to this concept). We only have conceptions of reality formulated by our experience as human beings with human brains, but we cannot be certain that what we view is actually true about the world or if it merely our misguided perceptions.
Another possible explanation: I believe what he is really trying to say (of course referring to Allegory of Cave) is that people grow up being exposed and taught in the world, and never being exposed to any other environment in the same way. Therefore we base and compare perceptions and experiences to those in the past, thus formulating our own reality; whether intentional or not.
(The cave is the brain, nothing other than your mind is for sure, as it’s all you’ll ever know and it’s contents are all you can compare things too)
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