Various versions of the argument from design have been around for a long time. St. Thomas Aquinas was the first to present this line of thought. His argument was very different from Cleanthes' version of the argument, in large part because Aquinas' version was based on a very different picture of the world. Aquinas believed in an Aristotelian teleological picture of the world, on which all phenomena are explained in terms of their ultimate purpose. (Every event and every object is assumed on this picture to have some larger purpose that it exists to fulfill.) Given this picture of the world, Aquinas reasoned that there had to be a divine creative mind, in roughly this way: (1) Natural bodies act for an end or purpose. (2) Natural bodies are not themselves aware of acting for this end or purpose. (3) Therefore, there must be some other cause that explains why they act for this end or purpose. (4) This cause is God.
By Hume's day the picture of the world had altered significantly. René Descartes and Sir Isaac Newton had changed the way people thought about the nature of the physical world, replacing the teleological picture with a mechanistic picture. In the mechanistic picture the world is seen as operating based on immutable laws of nature, and not for any ultimate goal. The new version of the argument from design developed in response to this change in worldview. Sir Isaac Newton was himself vociferous in defending this version of the argument from design (Descartes, on the other hand, tried to give a priori proofs for religious truths).
The new mechanistic picture of the world does not only play into Cleanthes' arguments, but into Demea's as well. When Demea claims that God cannot have ideas of sensation it is because he is heavily influenced by Cartesian and Newtonian science: according to these pictures (which anticipate the current prevailing views) the world around us is made up of colorless, odorless, tasteless, soundless particles of matter which join together in various ways. Our sensations, therefore, do not give us access to the world as it really exists; they are thus misleading. God, as he cannot be deceived, could not have any sensations. It is interesting to note that Cleanthes uses analogical arguments to defend his argument by analogy. He says that the case of the voice from the sky is analogous to the question of the creation of the universe, and then says that if the result of the application of Philo's rules is absurd in one case, it must be absurd in the other.