Some context: David Kellogg Lewis (September 28, 1941 - October 14, 2001) was an American philosopher. Lewis taught briefly at UCLA and then at Princeton from 1970 until his death. He is also closely associated with Australia, whose philosophical community he visited almost annually for more than thirty years. He made contributions in philosophy of language, philosophy of mind, philosophy of probability, metaphysics, epistemology, philosophical logic, and aesthetics.
Lewis went on to publish Counterfactuals (1973), which contained an analysis of Counterfactual conditionals in terms of the theory of possible worlds. He had already proposed it in some of his earlier papers: "Counterpart Theory and Quantified Modal Logic" (1968), "Anselm and Actuality" (1970), and "Counterparts of Persons and their Bodies" (1971). In spite of significant technical advantages promised by this approach, the theory was widely considered to be too implausible to be taken literally, as Lewis urged it should be. Most often the idea that there exists an infinite number of causally isolated universes, each as real as our own but different from it in some way, and that furthermore that alluding to objects in this universe as necessary in order to explain what makes certain counterfactual statements true but not others, meets with what Lewis calls the "incredulous stare" (Lewis, OPW, 2005, pg. 135-137). Lewis defends and elaborates his theory of extreme modal realism, while insisting that there is nothing extreme about it, in On the Plurality of Worlds (1986). Lewis acknowledges that his theory is contrary to common sense, but believes that its advantages far outweigh this disadvantage, and that therefore we should not be hesitant to pay this price.  According to Lewis, what makes a counterfactual conditional of the form  "Had I made that shot our team would have won the game."  true is that there is a world, as concrete as ours and significantly similar to it, in which my counterpart makes rather than misses the shot and the counterpart of our team wins the game. Had there been a world even more similar to ours in which my counterpart makes the shot but the counterpart of our team still loses the counterfactual would have been false. When we speak of counterfactual possibilities we speak of what is the case in some possible world or worlds. According to Lewis, "actual" is merely an indexical label we give to a world when we locate ourselves in it. Things are necessarily true when they are true in all possible worlds. (Note that Lewis is not the first one to speak of possible worlds in this context. Leibniz and C.I. Lewis, for example, both speak of possible worlds as a way of thinking about possibility and necessity, and some of David Kaplan's early work is on the counterpart theory. Lewis's original suggestion was that all possible worlds are equally concrete, and the world in which we find ourselves is no more real than any other possible world.)
What is modal realism?
A: the idea that there exists an infinite number of causally isolated universes, each as real as our own but different from it in some way,
Some context: Thomas Simpson (30 November 1937 - 13 July 1967) was one of Britain's most successful professional cyclists. He was born in Haswell, County Durham and later moved to Harworth, Nottinghamshire. Simpson began road cycling as a teenager before taking up track cycling, specialising in pursuit races. He won a bronze medal for track cycling at the 1956 Summer Olympics and a silver at the 1958 British Empire and Commonwealth Games.
Simpson was born on 30 November 1937 in Haswell, County Durham, the youngest of six children of coal miner Tom Simpson and his wife Alice (nee Cheetham). His father had been a semi-professional sprinter in athletics. The family lived modestly in a small terraced house until 1943, when his parents took charge of the village's working men's club and lived above it. In 1950 the Simpsons moved to Harworth on the Nottinghamshire-Yorkshire border, where young Simpson's maternal aunt lived; new coalfields were opening, with employment opportunities for him and older brother Harry, by now, the only children left at home. Simpson rode his first bike, his brother-in-law's, at age 12, sharing it with Harry and two cousins for time trials around Harworth. Following Harry, Tom joined Harworth & District CC (Cycling Club) aged 13. He delivered groceries in the Bassetlaw district by bicycle and traded with a customer for a better road bike. He was often left behind in club races; members of his cycling club nicknamed him "four-stone Coppi", after Italian rider Fausto Coppi, due to his slim physique.  Simpson began winning club time trials, but sensed resentment of his boasting from senior members. He left Harworth & District and joined Rotherham's Scala Wheelers at the end of 1954. Simpson's first road race was as a junior at the Forest Recreation Ground in Nottingham. After leaving school he was an apprentice draughtsman at an engineering company in Retford, using the 10 mi (16.1 km) commute by bike as training. He placed well in half mile races on grass and cement, but decided to concentrate on road racing. In May 1955 Simpson won the National Cyclists' Union South Yorkshire individual pursuit track event as a junior; the same year, he won the British League of Racing Cyclists (BLRC) junior hill climb championship and placed third in the senior event.  Simpson immersed himself in the world of cycling, writing letters asking for advice. Naturalised Austrian rider George Berger responded, travelling from London to Harworth to help him with his riding position. In late 1955, Simpson ran a red light in a race and was suspended from racing for six months by the BLRC. During his suspension he dabbled in motorcycle trials, nearly quitting cycling but unable to afford a new motorcycle necessary for progress in the sport.
What does his mother do?
A: