the concept that you are either alone in the world, and everything around you is a dream of your own creation, or, that as an entity/conciousness, you are completely cut off from everything other than your own perception of things which is fundamentally different than and alien to any other perception of entity in the world and that no1 can ever understand anything outside themselves. it also means that no1 can truly be sure that anything outside of one's own imagination has any existance or meaning.
Am I all that is? Or are there others, and each of us in an inescapable perceptual bubble? To start here (especially the latter) and muse about reality can be sound, I think, if it keeps one on his/her philosophical toes about their musings and conclusions. But solipsism is not the best "as if" to adopt if it leads to inaction, selfishness or dangerous behavior.
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Shinier than a speeding bullet.