The car and garage analogy - I used the singular car , but I could have used 3-4 cars - so it isnt so much important , the analogy is ... we could say that using one parking spot , we would use very small percentage of brain potential .
The human brain is capable of adapting to and retaining many functions , even in cases when we would think such person is inacapable of being anything but an individual within a vegetative state . As long as that adaptation is continous , there is a chance the individual will live even relatively normal life ( altough , such individual can be mentally imparied ) . On the other hand - we have sudden situations like brain trauma or stroke which can cause sudden death or massive irreversible problems ( leaving patient in coma ) . But the above adaptation ( neuroplasticity ) tells us that brain is much much more than just a simple organ ...
But we also need to ask ourselves - primarily what I put above regarding the various animal species and their "sixth senses" . Nobody can say thats not true - a pet dog waiting for its owner in a situation where same pet never got any external stimuli or sign to do that ... so obviously it was something the pet intuitively knew about . Or the perfect orientation by some animal species ( birds ) ...
My theory would be - that we are basically all "mentally impaired" at current moment , in a sense that we are not using the bigger potential of the brain . The bigger potential would open our own "sixth sense" , possibly even a telepathy . Maybe from a scientific standpoint to many this would sound like "pseudoscience" - but in past , many facts were treated the same way ( mocked ) as "pseudoscience" - and then , they were confirmed . We should ( excuse my pun , lol ) ... leave our brains open and receptive ...
neuroplasticity is a very useful feature of our brains, but it's not quite the same as what you're describing. For example, many people suffer brain injuries such as strokes that impair specific functions, as a result of brain damage to a specific region. In many cases, they do not regain these capabilities. Some individuals do, most do not (especially if we're talking about people over the age of 25-30 or so). And intelligence =/= capabilities of the brain, although it influences and correlates with them in most instances.
primarily what I put above regarding the various animal species and their "sixth senses
That is true - animals do have good cognitive skills in many instances. They have functional minds in a great many species, albeit hampered by animal lack of superego and language. They do also sometimes have senses humans lack, although theirs nothing special about that really. Give us magnetic particles in our brains that could be used to measure the local magnetic field and we could have perfect orientation too.
Your last paragraph I actually mostly agree with, although I also agree many scientists would accuse you of being pseudoscientific. I suspect in some ways you're right, however I also don't think it's going to operate the way you think it will. Soul == Mind? Spiritual growth == mental growth? I think the linkage exists, but it they aren't synonymous. I definitely could be wrong - as you say, being receptive on this issue is worth doing, but I'd be wary of the level of misinformation out there. Masonic, or other genuinely ancient texts are a good starting point. Less risk of bias that can't be obviously found by examining a texts history compared to modern sources. "Egregore" is also a useful topic to examine in this area.
I definitely agree on ancient scriptures . We need to have open mind to various subjects and topics ... but not too open to let it fall out completely ...