In Burial at Sea we learn that he and Suchong collaborated to make vigors, and they briefly mention Fink having to send crews down to Rapture to get slugs, but prior to the DLC, was there just no explanation? Even the one they came up with is pretty flimsy since Big Daddies would be guarding Rapture and the slugs would have been extremely valuable in the making of Little Sisters, not to mention that drinkable vigors require ten times as much ADAM as injectors.
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did you really play burial at sea and look at your surroundings, or did you just go through the motions?