Dimensions (beyond the 4 we know and the 10 we have theorized) are an interesting concept. I have a few theories on them myself (relating to life, death, God, etc.). However, I never considered imagination as being a dimension unto itself. if imagination is a dimension, then it must be partitioned (much like a computer hard drive), with each partition accessible to one mind. (There an infinite number of people - past/present, future - as each imagination/mind is unique. Thus, when one mind "dies", the partition its imagination occupied would either collapse or be forever "cut-off" (inaccessible). However, unless that dimension had unlimited "space" (physical?) it would have to grow smaller as time (in this dimension) went on (of course, who says it has to occupy the parameters of the first 3 dimensions?).
Ok, you’ve really piqued my interest here. I’ll spend days trying to wrap my mind around this.
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I can (and do) create "dimensions" in my imagination. Some of which would-be quite interesting (perhaps scary?) if they actually existed in reality. A few of those “dimensions” are universes unto themselves (in size and scope).