  On the basis of such observations, Heidegger argues that to be Dasein at all means to Be-with: “So far as Dasein is at all, it has Being-with-one-another as its kind of Being” (Being and Time 26: 163). One's immediate response to this might be that it is just false. After all, ordinary experience establishes that each of us is often alone. But of course Heidegger is thinking in an ontological register. Being-with (Mitsein) is thus the a priori transcendental condition that makes it possible that Dasein can discover equipment in this Other-related fashion. And it's because Dasein has Being-with as one of its essential modes of Being that everyday Dasein can experience being alone. Being-with is thus the a priori transcendental condition for loneliness.