As I mentioned earlier, Xehanort referred to the resistance of Terra's mind when the LS formed. As far as I know, there is little or no evidence to suggest that the soul contains a person's mind or selfhood. Sora and Xehanort retained the ability to think after becoming Heartless, and there's no evidence to suggest that their bodies or souls had any role in that. However, it has been confirmed that the strength of a person's heart decides whether they leave behind a mindless shell, a common Nobody, or a fully humanoid Nobody capable of speech.
Yen Sid: "At times, if someone with a strong heart and will - be they evil or good - becomes a Heartless, the empty shell they leave behind begins to act with a will of its own."
Notice the use of the word "will" as well. It seems to suggest that a Nobody loses both the heart and will of its former self, yet develops its own will [and heart].
In addition to all of that, we know that Nobodies retain memories of their previous lives - another component of the heart. Now, maybe it's just me, but it seems a little odd that the mind would be synonymous with or part of the soul - which has basically been said to act as an energy source for the body - and not a part of the heart, which contains a person's memories and emotions. Why not place the mind or will under the same category as the other parts of the psyche?