In examining these arguments, one must admit that Brett makes a good point. There are indeed no fossil records, ancient accounts, or indeed current evidence from our considerable technological searching devices to suggest that worlds beyond our own exist. Once again, however, evidence in this regard pertains only to one type of reality -- our own. The second part of the Lewis's assertion, as quoted above clearly indicates that the philosopher is not considering other possible worlds only in our universe and our reality. Instead, he appears to consider the existence of possible worlds as indeed being possible across a wide spectrum of realities. The fact that they are not part of our reality makes them no less real.
When the apparently paradoxical statement regarding other worlds is taken into account: "They are not far in the past or future, nor for that matter near," Lewis is clearly not referring to worlds that we can observe with physical scientific instruments. These worlds are apart from our own not only in physical reality, but also in terms of space and time.
The problem then becomes one of scientific deduction. How does Lewis know that these worlds exist? What are his premises for believing in the existence of such worlds? Lewis himself admits that he does not know very much about these worlds, nor how to find out more. He appears to base his philosophical system upon the observations of modality in our own reality. The fact that possibility, choice and there modality exists in our world and our language leads to the conclusion that other worlds must exist, where creatures such as ourselves make different choices or evolve differently to create a vast number of other worlds.
Each world is separate from the others in terms of space, time, and reality. However, the fact that we speak of our reality as the only one only means that it is the only one for us. We cannot access the other realities by means other than thought. There is no way to communicate with our counterparts in those worlds.
It is therefore not difficult to understand why critics such as Michael Brett so adamantly disfavors Lewis's premise. Indeed, Brett takes Lewis's premise quite literally, as apparently the latter intends him to. The main fallacy in Brett's observations is however that he applies it in terms of only a single reality. He does not or cannot accept, as Lewis apparently also intends the reader to, that realities other than this space and this time exist. He applies all Lewis's observations to a single reality, which then leads to the conclusion that Lewis must be incorrect.
I do not however believe that Lewis would regret or even resist such refutation. Indeed, being a philosopher in the true sense of the word, Lewis accepts that there would be viewpoints other than his own, and even welcomes these. Indeed, in his introduction, he urges the reader not to accept any of his assertions without critical thought, nor to assume that the author believes that his premise is the only possible one. Indeed, he appears to also apply his theory of possibility, either consciously or subconsciously, to his own philosophies. As a believer in infinite possible worlds, of which he is a single of many possible inhabitants, Lewis observes that his thought is but one direction of many possible others. He also admits that he could have missed some important points in evidence for or against his premises.
Having said this, however, the philosopher offers his readers much food for thought. His observations and deductions are based upon philosophical rather than scientific evidence. There is no physical evidence for the infinity of worlds in Lewis's "multiverse." However, he argues convincingly for their likelihood on the premise of the existence of modality in the human reality as observed by scientists and philosophers alike. Personally, I feel that the existence of such worlds is infinitely, so to speak, more appealing than the possibility of only one reality. Indeed, the very existence of possibilities themselves suggests that these worlds do exist, although no way has yet been established to make contact with them.
Brett, Michael. Review of on the Plurality of Worlds. In the Philosopher. Retrieved from http://www.the-philosopher.co.uk/reviews/lewisworlds.htm
Lewis, David. (2001). On the Plurality of Worlds. Oxford: Blackwell Publishing Ltd.