Let us say the proposition is put that “the Loch Ness Monster exists”. The response of some individuals is immediate and negative. We may then ask, how can you be certain of its non-existence? More fundamentally, what is it that you know does not exist? Thinking reflectively, have you had the necessary information and have you defined the problem sufficiently to come to such a certain conclusion? If not (and this is probably the case) have you made similar judgments about other more important matters, where you are equally ill informed?
Some individuals may be equally positive. Again, we may ask on what basis they make the judgement. Typically this is an accumulated feeling based upon media coverage and a liking for such matters. This latter should not be dismissed, but recognized as part of human nature. People who would like something to be true are more likely to interpret ambiguous evidence in that way. Likewise people (including scientists) tend to be more critical of unwelcome data.
Some individuals will claim to have an open mind. Very well, we may ask what would enable them to come to a definite belief? If this was a grave matter of public health or of great financial risk, and you had to go one way or another, how would you choose?
Firstly it is necessary to decide what it is that we are trying to conclude. The following is suggested as a rational and minimal basis for discussion.
“That in the 20th Century there has been a number of large animals living in the waters of Loch Ness.”
Thus we are concerned with natural history rather than supernatural, but we should include the possibility that a small breeding herd has died out. Present non-existence would not disprove reports from earlier years. By large, we may say ‘more than 2 metres in length’. Although witnesses have claimed much larger (20 m) all that is required is something significantly larger than the otters and eels known to be in the lake. (Would you accept a giant otter?)
It is not necessary to include all the features which have been reported (head, neck, flippers, humps etc.) since some of these could be mistakes or embellishments. It is probably reasonable (or is it?) to say that the minimum requirement is a large aquatic animal. (so in simple terms its not known for certain if the loch ness monster exists)
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Even if one million of scientists will have proved that there is absolutely no mysterious animal in the Loch Ness, people will still talk about it and keep speculating and sometimes they will still see and photograph it. 
