Software represents the middle ground between what we are thinking and what we want our machines to be thinking. It is a bridge between our intentions and reality. It is a framework for a world we first imagined and then dragged into existence from the void.
Source code should be accurate and informative for both humans and machines. It is a message to the future about what was in our minds -- even if the recipient of that message may be ourselves after we've forgotten a few things; or someone we've never met trying to unravel a mystery, gain some new perspectives on their craft, or simply nudge their reality in a new direction.
Software should make sense and dissolve confusion.
Software should provide leverage without unnecessarily obscuring the truth.
Software should do what it says it does; and to the extent possible it should do so reliably.
When software fails it should tell us why it failed in a meaningful way so that there is no mystery.
Software should provide a foundation for the future. It should be flexible and reusable so that it can be continually improved, re-purposed to new tasks, or borrowed as a template for something entirely new. It's pieces should be stepping stones to new abilities that perhaps were previously unimaginable.
Software should not only serve a specific purpose, it should go beyond that to say something about how that service is achieved, what may be possible, and the musings that went into it's creation. It should be more than bits and pieces. It should reveal something of "the ghost" in the machine.
Software should be beautiful and elegant. Though it's encoding and principles are grounded in science, it's form should aspire to art.
With each line of code we inject a bit of ourselves into our work. That part of us lives on and represents us. Our intentions are bound in the actions derived from the paths we lay down in source code's structure. A bit of our spirit is imparted to each effort we make toward it's completion. In it's own way it speaks for us when we are gone.
It should always embody our best effort.