The master’s eyes darted around, and he continued in a low voice:

When we do our work poorly, we are replaced with our betters. When we do our work well, the thing we have built grows larger, faster, more powerful, more entrenched, more hungry. Sometimes I lie awake in a cold sweat, unable to decide if we are still building it, or if it has begun using us to build itself …

(Link received with gratitude from JF — a more precise attribution might be unwelcome.)