In ‘The Turning Point in Philosophy’ Moritz Schlick expressed the following view: ‘Everything is knowable which can be expressed, and this is the total subject matter concerning which meaningful questions can be raised. There are consequently no questions which are in principle unanswerable, no problems which are in principle insoluble.’ (page 56 in Logical Positivism edited by A. J. Ayer.) I will refer to this as Schlick's principle, although it is shared by many others. What it amounts to is the view that all meaningful questions can be answered by rational procedures, that is by logical argument or appeal to evidence. It is this view which I wish to challenge, at the same time relating what I have to say to belief in God. For it follows from Schlick's principle that provided the question ‘Does God exist?’ can be meaningfully asked, it is in principle possible to answer it either affirmatively or negatively. Having done so, we would then know, and not merely believe, that God does or does not exist. I will try to show that provided the question ‘Does God exist?’ can be meaningfully asked, no such consequence need be taken to follow.