What is x?
It is the interface between Thinking and Feeling.
As an attribute, it is an attribute of the ego, an extension of the ego, the attribute we call self-awareness.
Self-aware is something we as human beings can be and animals cannot.
Self-awareness as a capacity is something we should develope in ourselves, which is to say it is a psychological characteristic we should value.
For more see discussion about Psycho-epistemology & Psycho-hermeneutics.