Board Thread:General Discussion/@comment-27907368-20160528162922/@comment-27620479-20160628055736

I think I've found a probable answer to why can monsters absorb human souls, but can't take determination injected into them.

Alphys doesn't talk about physical matter needed to hold certain amounts of determination. She talks about physical matter needed to hold certain concentrations of determination. It doesn't answer why monsters have less determination to begin with, but that is just a trait, and doesn't mean they couldn't have/take more. Now, we know souls are more than just determination. They hold other traits, most notably compassion, but also one or more of patience, bravery, integrity, perseverance, justice, kindness, and probably more.

When a monster absorbs a soul, they take determination with whatever else there is. They gain high amounts of determination, but not necessarily high concentrations. Also, their body changes into that of a "horrible beast", probably to accomodate their new powers, and the higher amount of determination. In Alphys' experiments they only gained determination, increasing both amount and concentrations, as they didn't get the other traits with it, at the same time.