Star Trek: 10 Things You Didn’t Know About The Guardian Of Forever
9. It Is Both Sentient And Alive, While Also Being Neither
In many ways, the Guardian of Forever could be considered fully sentient and alive. It has the power of rational thought and is aware of its nature and surroundings. Along those same lines, it could be a being, as opposed to a machine. However, the Guardian itself says that it doesn't consider itself to be either a being or a machine, while it also considers itself to be both.
This cryptic answer is offered up to Kirk and Spock when they first encounter the Guardian in The Original Series. The depiction of the Guardian in that episode is something of a cold observer of time - it has grown so old that the conventions of a few hundred years are both meaningless and something to be played with.
Even when time is set right again, it offers those who changed it the chance to do whatever they wanted again - something unspeakably dangerous. That said, by the Animated Series' episode Yesteryear, Starfleet has returned to the planet to study the Guardian again. A similar mishap occurs, which should be proof enough that this Device-Being should be a little more choosey over who it allows to operate it.