What is y?

It is the interface between Thinking and Acting.

As an attribute, it is an attribute of the ego, an extension of the ego, the attribute we call self-assertion.

NOT asserting the self is something we human beings can do but animals cannot. We have volitional control over assertiveness, animals do not.

Our life requires Action to sustain it, therefore we should be assertive.

Self-assertion as a capacity is something we should develope in ourselves, which is to say it is a psychological characteristic we should value.