Present-to-Hand and Ready-to-Hand

Heidegger describes two ways of relating to tools: one when they're working, one when they're not.

When a tool – a hammer, say – is working well it disappears from our consciousness. We're not relating to it; rather, we're focused on our task. It is "ready-to-hand." Dawn and Ready-to-Hand

If something goes wrong with our unconscious use of the tool (as when it breaks), it becomes "present-to-hand. We now think about the tool as an object, composed of interrelated pieces. We *diagnose* it.

Heidegger is not an easy read, but here is a nice description.