For Purely Scientific Purposes

 is a mission in Red Dead Redemption.

Story
Nastas has set up a meeting with some of Dutch's men, for unknown reasons. Marston will accompany the Professor and Nastas to what turns out to be a set-up, where Nastas ends up dead and Marston has to fight for both his own and MacDougal's life.

Walkthrough
Get on a horse and follow Nastas and the Professor to Bearclaw Camp. About halfway there, a grizzly bear will cross the road, which the professor will open fire at. The grizzly turns hostile, and you'll have to kill it. After taking care of the bear, move along until you reach the final destination.

Once you reach the cabin, you'll meet with three of Dutch Van Der Linde's men. They pull their guns at Marston and his companions, in spite of the Professor repeating that they're there in peace. When Nastas tries to talk to the indian, he gets shot in the face. Marston will pull his gun and kill all three men. The men outside will turn hostile, and Marston will have to kill them all.

Once you've killed them all, get on your horse and follow MacDougal back to Blackwater. You'll be attacked by two groups of Dutch's gang, on horseback. Take them out and accompany MacDougal to his office.