Make sure your thesis is specific & concrete

Amber Crofts writes:

Mr. Duncan,
Thought I would run this by you:

Those w ho believe in the torch and sword, including Marlow, learn that
their fate is not up to them and can change through the experiences
they encounter in the Congo.

See you tomorrow,
Amber

Dear Amber,
The problem with this thesis is that it's too vague. It's the way you finish it: "their fate is not up to them and can change through the experiences
they encounter in the Congo." You must spell out what their fate is and what those experiences are. Expressions like "can change" leave too much room for doubt and speculation, and make your path to a conclusion more difficult for you.
What do you want to say about "the torch and the sword"? Is the concept good or bad, real or false?
You were interested in Marlow's fate and Kurtz's fate and Fresleven's fate. All of them undergo internal change, spiritual change. Kurtz shucks his principles for ivory and supremacy. Fresleven goes from gentle and quiet to exasperated and brutish, and dead. Marlow's faith is shaken—perhaps destroyed—along with his health. Something infects them all, and it all starts in that creepy office in the "sepulchral city."
Your thesis should be strong and specific, and should lead to an informed discussion of these issues.
JD