developed a variety of enemies that would have certain strengths and weakness that would force the player to experiment and work with all their available tools given to them. They also wanted moral choices in the game to be less obvious and with initially ambiguous results. Early versions of the games had included Nostrums as gameplay elements. Nostrums would have made permanent changes to the character and could not be removed once used. The Nostrum system was scrapped in favor of using gear, as they found that the system was too similar to other ones that the player would have available in equipping their character, compared to the gear option. Similarly, earlier demonstrations of the game did not use Salt to power vigors, but instead each vigor would have a limited number of uses before it was exhausted. While more could have been found around the game's environment, the player would have only been able to carry a limited number of vigors into battle, with more powerful vigors containing fewer charges. In early builds of the game, as many as 16 enemies could be on-screen at once, but this was dialed back to just six, a constraint which impacted the game design. Now, each individual enemy had to be tougher, creating "bullet sponge" enemies. The AI for Elizabeth was a major technical challenge for Irrational; they wanted her to display realistic-feeling behavior in contrast to games where such companions are either highly scripted or are simplistic. The developers used the AI routines of the roving Big Daddy and Little Sister characters from BioShock as a starting point for improving Elizabeth, giving her the tendency to look and move around like a real person instead of a robotic non-player character that either fought the player or did nothing. The developers looked to the banter between characters in the Uncharted series by Naughty Dog as an inspiration. Elizabeth's responses are driven by a player's action or are independent of them, with the levels filled room by room with elements that Elizabeth could react to. Actions are performed based on a number of factors, including whether the player is near and looking at Elizabeth, and the current tension of the game. The game also monitors the player's actions to try and keep Elizabeth out of the line of fire. Cutting Elizabeth from the game was considered during development because of the difficulty in