All 3 Uses of

hypothesis
in
The Screwtape Letters

For real resignation, at the same moment, to a dozen different and hypothetical fates, is almost impossible, and the Enemy does not greatly assist those who are trying to attain it: resignation to present and actual suffering, even where that suffering consists of fear, is far easier and is usually helped by this direct action.
Chpt 6 *

hypothetical = something temporarily treated as true

Hypothesis after hypothesis has been tried, and still we can't find out.†
Chpt 19 *

Hypothesis after hypothesis has been tried, and still we can't find out.†
Chpt 19

Definitions:

(1) (hypothesis as in: a study to test her hypothesis)
a seemingly reasonable, but unproven idea or explanation based upon known facts

In casual conversation, theory is a synonym for hypothesis, but a scientist would say that a hypothesis needs to pass rigorous tests before it could be accepted as a theory.
(2) (hypothesis as in: assume as a working hypothesis)
something that may or may not be true, but is temporarily treated as true to advance a discussion or to further investigation

This sense of hypothesis is typically seen in the form, hypothetical, or hypothetically.