"The adoption of Brennan J's notion of a reasonable hypothesis
meant that Parliament was requiring something by way of a causal
link, but which fell short of proof of the link - even prima facie
- as a fact. The meaning of the phrase 'reasonable hypothesis'
was felicitously explained by a Veterans' Review Board in Stacey
(unreported Nos V83/0396, V84/0821 and V28/072, 26 June 1985);
words quoted by the Administrative Appeals Tribunal in Re Dell and
Repatriation Commission (1986) 5 AAR 253 at 254-255:
'A hypothesis may be conveniently defined as: "proposition
made as basis for reasoning, without assumption of its
truth; supposition made as starting point for further
investigation from known facts; groundless assumption": The
Concise Oxford Dictionary.
...
The addition of the word "reasonable" would however seem to
imply that what is required is more than a mere hypothesis.
In the opinion of the Board, to be reasonable, a hypothesis
must possess some degree of acceptability or credibility -
it must not be obviously fanciful, impossible, incredible or
not tenable or too remote or too tenuous. For a reasonable
hypothesis to be "raised" by material before the Board, we
think it must find some support in that material - that is,
the material must point to, and not merely leave open, a
hypothesis as a reasonable hypothesis. At the same time,
however, a hypothesis may be reasonable without having been
proved (either on the balance of probability or beyond
reasonable doubt) to be correct as a matter of fact. Were
it otherwise, it would no longer be a hypothesis but would
have been elevated to some higher status. Accordingly a
connection asserted by a hypothesis to exist between death
or incapacity and service may still be reasonable even
though theoretical, and it may be theoretical in either or
both of at least two senses: by postulating a known medical
fact but in circumstances not known to have definitely
existed in the instant case; or by postulating a medical
principle which science is not yet able to definitely prove
but is unable to describe as unreasonable.'
We agree with this analysis. A reasonable hypothesis requires
more than a possibility, not fanciful or unreal, consistent with
the known facts. It is an hypothesis pointed to by the facts,
even though not proved upon the balance of probabilities."
(pp 532-3)