"For years I have been mourning and not for my dead, it is for this boy for whatever corner in my heart died when his childhood slid out of my arms"

In this poignant quote by William Gibson, the speaker reveals a deep sense of mourning that goes beyond the traditional sadness associated with the death of a loved one. Rather, the grieving is for something more abstract and deeply individual-- the death of a stage in life encapsulated by the development of a kid. The "kid" referred to in the quote is most likely the speaker's boy or a child near their heart, whose growing up symbolizes an irreversible modification.

The speaker reviews the bittersweet shift from youth to the adult years and the resultant sense of loss. This is not just about the kid maturing but about the parts of the speaker's own identity and experience that have actually changed as an outcome. The expression "whatever corner in my heart died" recommends that there belonged of the speaker's psychological world that was elaborately tied to the kid's developmental years-- a part that offered pleasure, function, and perhaps a sense of marvel. As the kid grew and "slid out" of their arms, these elements likewise faded, leaving a mentally vacant area within the speaker.

Using "mourning" indicates an extensive, enduring sorrow, equivalent to the experience of losing someone through death. However, it is the intangibility of this loss that contributes to its intricacy. There's an implicit recognition of the inevitability of modification and the approval that time can never ever be unwound. Yet, this approval doesn't make the experience any less agonizing or significant. The passage captures the essence of parental love and the universal experience of letting go, conveying a deep, human truth that is relatable to anybody who has viewed a child mature.

In general, Gibson's quote eloquently articulates the duality of love and loss and how they often exist simultaneously, highlighting how personal development can resonate with the same depth as more widely acknowledged forms of grieving.