Light is not necessarily "souls" - that is extreme conjecture even for this sort of probing, which is otherwise pretty solid.
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For one of the exotic bounties, you have to kill over 100 enemies to charge your empty shell of a gun (Am I thinking of Invective?) And I'm pretty sure it says it needs light to be charged. Seeing as kills was the only way to charge it, safe assumption is that Light is, for lack of better description, a soul