I now noticed about the difference between those sentences.
The first one is close to:
(I think that) the substance is clear enough. (c)
(I think that) I can understand the substance well. (d)
The second one is close to:
I think that readers would be able to understand the substance well. (e)
The (b) can mean (c) and (d), but I guess that it is closer to (e). The subject who understands sentences is different.
http://lang-8.com/114231/journals/898827/ (Not mine)
http://lang-8.com/satoshi/journals/958178/ (Does 思うと思う make you confused?)