You completed Hitman: Absolution as a professional

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There are 5 total difficulties to choose from:

Easy
Normal
Hard
Expert
Purist

Complete each mission on Hard or higher without changing the difficulty. Pick one of those difficulties and stick with it until you have the achievement. All three are considered "professional". You have no player activated checkpoints and enemies have better than average reaction times. Also your instinct won't restore on it's own.

You always have the option to replay missions on easier settings. The game always asks you what difficulty you want to play the missions on. If you play the entire game on Normal, you can just replay missions on Hard to gain the achievement. You do not have to restart the entire game. Replaying missions will work.