Duncan can firmly attest to this. Initially, Duncan hadn't been very confident that this technique could help him. Even though the description said it could strengthen the soul, Duncan was a bit suspicious about it.
It was only natural. After all, anything that had to do with the soul is not something that could be obtained easily. Furthermore, Duncan's knowledge regarding the soul was truly at the basic level. How can he be sure that his understanding of the technique was right?
To make things worse, his first try with a method mentioned in the technique caused him bone-deep fatigue. Even though the method succeeded, Duncan was hit by the side-effect. So, Duncan wasn't sure there were any more side effects that he needs to be aware of.
In such a situation, it is understandable that Duncan wasn't highly favorable towards this particular technique in his heart.
