OK, I will bite. I agree with him. If there is insufficient evidence in favor of the existence of the Loch Ness Monster, then I, as an intelligent and rational person should come to the conclusion that there is no Loch Ness Monster. So, yes, with insufficient evidence in favor of a given proposition, then you should probably not believe it.
Does your conclusion that the Loch Ness Monster does not existence prevent acknowledgement of the possibility that new evidence might come to light in the future?
If this
is prevented, does it mean that if new evidence were to come to light you would be unfairly sceptical of it (since you had written off such a possibility you would be more sceptical than a person who had not decided either way)? Does it mean that if the Loch Ness monster were later shown to exist, you would have held an incorrect belief?
If this is
not prevented, then do you
really believe that the Loch Ness monster does not exist since you are accepting the possibility that it might?
I realise this is more of a semantic challenge but I am curious as to whether in your mind, there is no possibility that the Loch Ness monster exists or whether you accept such a possibility (as vast and distant but nevertheless a possibility) yet still hold that your conclusion is that the Loch Ness monster does not existed.
My reason for my curiosity is I do not understand how such a possibility can be accepted (and indeed it seems as if it must be accepted due to the very nature of sceptical argument) and yet such a conclusion still be found. The two appear contradictory to me.
I speculate that what you are actually saying is that since the possibility is so distant, you may as well live your life with the assumption that the Loch Ness monster does not exist since, if it did, you cannot detect it and so it makes no difference to you.
Please note that this is not in fact what Clifford is saying in this quote (although he may very well have agreed with you). He is not saying that, given a lack of evidence, it is okay to assume the opposite. He is saying that, given a lack of evidence, belief in that thing is immoral. The other alternative would be to refrain from making a conclusion. I am not familiar enough with the man and his beliefs to say which of these he is advocating as the "moral" choice.