"We must be ready to do battle against an alien force."

The woman who lived in this building was, I believe, a conduit for one or other of those alien forces.

And yet, I ask you, is not an alien force already among us?

But now, again, came that awful sense of being occupied by an alien force!

An alien force, registered only in the consciousness, made itself felt.

I seemed now always to be discovering alien forces of character in her.

In the 23d century, astronauts from Earth contend with alien forces.

Some alien force, projected onto all of us, pushed out by them.

But here is no alien force, only that which we know in part.

For the third time in the 23d century, astronauts from Earth contend with alien forces.