upon one's guard

Is it possible, under such circumstances, always to be upon one's guard?

If one don't mind what one's about, one is blown up in a moment; and to be always upon one's guard, is worse than ruin itself.

One i'n't always upon one's guard.

I would not show that we are at all uneasy, doctor; only be upon one’s guard.”

It is well to be upon one's guard.