He points out that we know that design is a cause of order only in human activities, and these are a tiny part of the universe. Can we make such a small part a rule for the whole? He suggests that since the universe taken as a whole is a special, unique case, we simply have no relevant experience at all from which we could infer its cause. For himself, he says, so far as reason goes, he is content to say that he does not know what is the cause of the ordered, structured and purposeful world.

Hume’s introduction of his own epistemology into the Dialogues as a critical weapon can be seen also in the case of Demea. From Part II to Part VIII, Cleanthes’ design argument is thoroughly examined. As a result, it appears full of difficulties, and open to many doubts. Demea thinks such an argument, which Cleanthes asserted to be the only possible way to establish results in natural theology, is wholly inadequate as a basis for religious belief. In its place, he offers a ‘simple and sublime argument a priori’ which will be ‘infallible’. This is a version of what is known as the cosmological argument. Whatever exists must have a cause or reason for its existence. In considering the series of causes of things, we must either think of this as going on in infinite succession, or we must think that there is an ultimate cause whose existence is necessary. But there cannot be an infinite series of things each of which is caused to exist by its predecessor and causes its successor to exist. For, if we consider the infinite series as a whole, it too must have a cause for its existence. (There could have been nothing at all rather than the whole infinite series.) Ex hypothesi there is no external cause for the whole infinite series. Nothing which is a member of the series can cause the whole. Consequently, it has no cause for its existence, which contradicts the premiss that whatever exists has a cause for its existence. ‘We must, therefore, have recourse to a necessarily existent being who carries the reason of his existence in himself; and who cannot be supposed not to exist, without an express contradiction.’

This time it is Cleanthes who refutes the argument, employing Hume’s principles. The essence of the refutation rests on the distinction between relations of ideas and matters of fact. Whether or not something exists is a matter of fact. Anything which is provable a priori is such that its negation is inconceivable. But whatever we think of as existing, we can equally think of as not existing. Therefore, there is nothing whose non-existence is inconceivable, and so nothing whose existence can be proved a priori. In fact, ‘the words… “necessary existence” have no meaning’. What Cleanthes says here is derived from Book I, Part III, Section VII of the Treatise. There Hume says:

‘Tis evident… that the idea of existence is nothing different from the idea of any object, and that when after the simple conception of any thing we wou’d conceive it as existent, we in reality make no addition to or alteration on our first idea. Thus when we affirm, that God is existent, we simply form the idea of such a being, as he is represented to us; nor is the existence, which we attribute to him, conceiv’d by a particular idea, which we join to the idea of his other qualities, and can again separate and distinguish from them.18

When we form the idea that 16 is the square of 4, we have a complex idea which contains the component ideas of 16 and the square of 4. By reflecting on this complex idea, we see that the components must stand in the relation they do. But, if Hume is right about existence, when we think of something, and when we think of it as existing, there is no additional component idea of existence which is related to the idea of the thing. Consequently, it cannot be that by reflection we see that the idea of existence must be connected with the idea of the thing, because there is no separable idea of existence. Hence the words ‘necessary existence’ cannot stand for an idea. They have no meaning.