How can you be so sure?

2 seconds sound clip from the Isle of the Dead (1945) movie soundboard.

You can hear this line at 00:42:01.052 in the Blu-ray version of the movie.

Quote context

[...]

- But until I do know, I must keep you away from the others, and if necessary, I'll make an end in the only way that we know that a Vorvolaka can be killed.

- It's impossible. I'm flesh and blood. I remember my mother and father.

- Yes, but when you awaken in the morning, what do you remember of the night before? Of your visits to the Englishwoman?

- Sometimes at night, I tend her. I help her when she feels ill. I loosen the nightgown at her throat.

- At her throat. That you remember.

- Can you remember dreams? Can a Vorvolaka in her human form remember the evil that she did at night?

- How can you be so sure?

- This much we know: You walk at night. You arise in the morning refreshed and filled with life, and your friend, every day she goes paler, listless, weaker.

- It is best that you stay away from the others.

- I hope that I'm wrong. I hope that everything that Kyra said is untrue, but until I know, I must watch you.

- Were you ill before?

- Before? Before I came to work for you?

- Yes, I was ill for a long time before.

[...]