Ayer admitted that a distinction between practical verifiability and verifiability in principle. There were many things which were meaningful at the time Ayer was writing but which had not been absolutely verified e.g. were there mountains on the other side of the moon. However, one day a means might be devised of finding out whether this was the case, so Ayer allowed that the idea was verifiable in principle.
Ayer later introduced two forms of the verification principle in recognition of the fact that there were also other types of statements which we accept as meaningful but are not absolutely verifiable, such as, scientific laws and generalisations like ‘arsenic is poisonous’. This could not be absolutely verified until every bit of arsenic in the world had been tested. The same applies to historical statements. Accordingly Ayer distinguished between the weak verification principle and the strong verification principle. Strong verification is when there is no doubt that a statement is meaningful, as it can be conclusively verified from our sense experience. Weak verification applies to those statements that could not be verified in practice, such as a scientific laws (for example, all metals expand when heated) or a general assumption (such as, all leopards have spots), such statements are still meaningful because we know what sense experience would count towards verifying them, experience renders them probable. Nevertheless, for logical positivists, to speak about anything over and above the physical world, is, they claim, ruled out by the verification principle. To talk about God existing may look on the face of it like a factual assertion but God cannot be seen, touched, or even smelt. When people talk about God doing this or that, what they in fact observe is not God it all, some physical occurrence or event involving human beings.
A major strength of the verification principle is that we often use it in everyday life, if someone says its raining outside then we automatically use our senses to verify that statement. Also, on the face of it, logical positivism is right to point out that for many people talk about God can be puzzling. People can make sense of statements like ‘I spoke to my friend yesterday’ and ‘the prime minister is the leader of this country’ but many people find it much more difficult to understand what is meant by statements like ‘god is in charge of life’ or ‘god speaks to me’.
However there are multiple weaknesses to the verification principle for example, why should we accept the meaningfulness of the verification principle itself? The theory itself is not verifiable. You cannot prove through sense experience the statement that a sentence can only be meaningful if some sense experience can count in its favour. Some scholars have claimed that the verification principle is far too strong since it not only outlaws religious language but it also makes much of what humans speak and write about as meaningless as well, including art, beauty and our inner feeling and sensations. For example how can we prove that the Mona Lisa is beautiful? The verification principle also makes poetic and metaphorical language meaningless.
The verification principle claims that a sentence can only be meaningful if some sense experience can count in its favour, however, how far can we trust our senses or the world of sense experience? Both Plato and Kant did not believe that the senses alone can be relied upon when forming judgements. Furthermore, our senses can often misjudge experiences under extreme circumstances e.g. mirages in a desert. As a consequence of this, many philosophers would argue that pure reason is a more reliable guide in verifying what is meaningful than our subjective senses.
Is talk about God really meaningless? Ayer himself admitted that some historical statements are meaningful even though they are not always verifiable e.g. Christopher Columbus landed in America in 1492. This implies that you don’t have to be able to verify or prove scientifically that a god exists to be able to talk meaningfully about such a being. Similarly, you can argue that although statements such as ‘there is life after death’ and ‘the bible is the word of God’ cannot be verified they still have meaning for believers. They may not be able to prove them via the senses but the statements are still meaningful to them.
Why should religious statements be treated like other statements? By their very nature religious statements are special because they refer to another sphere of existence, on this view the verification principle can only be meaningfully applied to material objects. John Hick has argued that religious statements can be verified eschatologically (at the end of time). God’s existence could be verified by each of us after we die and go to heaven.
The verification principle argues that religious language is meaningless, however with all its weaknesses it is difficult to say that it can be applied to religious statements and suggests that religious language is in fact meaningful. But this is difficult to accept for non-believers as for them, a lot of religious statements cannot be verified and so are meaningless. Perhaps a better way to conclude whether religious language is meaningful or not is to approach it in the way that Wittgenstein does in that religious language is meaningful to believers but less so to non-believers.