I am looking for a non-slang, non-colloquial word - a word that I can use when speaking to a professional therapist/counselor, to be exact.
Another way to ask this question might be "What quality does a solipsist (or maybe an egocentrist) lack?"
As an example, a young child steals a bicycle from another child. The child's parent asks the child why he stole it and the child replies "because I like it." The parent asks the child "Who else liked it besides you," and the child can't come up with an answer. What quality does this child lack?
This applies to many adults, but it's easier to recognize in a child.
At first I thought of "empathy," and that is correct at an emotional level. But I'm particularly interested in this at a rational level - an inability to understand another's thought process, available data, cultural influences, etc. and understand why a particular conclusion, decision, or action seemed right to him or her. So, the intellectual or rational equivalent of empathy (or possibly compassion) might work here as well.
Finally, I do not mean the inability to agree with or support others' viewpoints, only an inability to understand why they understand things as they do. This may sound like a lack of adequate analytical skills, but I know people like this who are otherwise quite capable of analysis and reason.
I am most interested in what quality such an individual lacks, but I secondarily welcome descriptive words for the individual other than "egocentrist" and "solipsist."