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you don't need to have income that exceeds your needs before investing into bitcoin. If you are waiting till you have excess income that's above your needs before investing, you might end up being unable to invest at all because needs will keep coming whether you like it or not.
By definition, you need to have either income or some side amount of savings (or other investment) that is greater than your expenses in order to be able to invest. If you do not have excess, then you are gambling rather than investing.
Of course, you could also invest if you don't have extra income but you are expecting your income to increase, but that would be a bit risky also, or you can invest based on future income that would be considered a loan, which is also risky and problematic if you don't already have funds to be able to service the loan and strong chances that your future income is not going to dry up.
If you are using the anticipation of paying the loan from the appreciation of the asset that you are buying, then you are gambling.. which may well be ok. if you have back up funds to service the loan, otherwise if you don't then you are likely taking excessive risk, which is also gambling. Sure people can do whatever they want, and sometimes gambling techniques work, but each of us should be careful if we are employing techniques that might be difficult to recover from if the price were to move against us or other unexpected things happen to cause us to lose our principle.
Is quite surprising that most people do not realize they are gambling thinking that they are investing because gambling is not just by playing sports betting but is also comprises of both investment because as an investor no matter how much they would loves to invest on Bitcoin but is never an option to borrow money and invest on Bitcoin when the person doesn't have any source if income that would help and sort out the Dept, however most people have gone into investment on Bitcoin through the money they had borrowed with the intention of paying back from the profit they will make, so you are right @JAY, however is unfortunate that some of the mindset of people concerning Bitcoin is totally wrong because most of them feels that investing on Bitcoin is a get rich quick investment perhaps is because of the way the price moves that's why most people feels that if they borrow money they could easily pay back through the profit they will realize without knowing that sometimes the price could go the opposite direction, so like you said borrowing money for investment with the intention of paying back through the investment is not a wise decision.