In a political context this is perhaps the most controversial issue; does the person in power actually have the power they are laying claim to? Did they actually win the election? In the second case, however you boil it down, he did win, the courts upheld the decision and however upset this fact made people, they upheld the rule of law, which in its own way made the presidency legitimate.
Throughout much of history, though, the question has been two-fold. Does the person have the power they are claiming and to whom was that person born? The answer to the first often stemming as much from the second as from any other source.
Such was the case at Philip II’s wedding when the guardian of the new Macedonian wife stood up and proposed a toast that Philip could now get down to creating a legitimate heir. While there is no evidence to suggest that Alexander was somehow tainted by bastardy, or even that such a concept existed beyond recognizing that some children were born of more important women; yet there was some notion of an ethnic identity and that at least some people saw or wanted to see that Alexander was outside of it.
Of course Macedonia had no rules about monogamy other than that you must be able to provide for your women and children (at least that was the idea), and in fact Kleopatra was Philip’s sixth wife, and the only one from Macedon. If he and his other primary advisors truly believed in purity of Macedonian blood, then he would have chosen a Macedonian wife earlier in his reign, rather than so late. The incident, too, must be regarded carefully in that Alexander had been considered the heir for the previous 18 years and only now, by the man who had the most to gain from Kleopatra’s child, was he called an illegitimate child. When Philip died shortly thereafter, Alexander promptly became king, receiving acclaim from the army and from a large portion of the nobility, even from Lower Macedonia, wherein true legitimacy theoretically resided.
Of course there were two additional factors at play. First, Attalos could not have been alone in his perception as his words did find receptive ears and at the same time struck home with Alexander. It was not an unknown bias, if not a majority one. Second, Alexander suffered residual resentment from his mother, who, by all accounts was a psychotic lunatic. So too were the rest of Philip’s women (Kleopatra aside), but that is another matter. Olympias went above and beyond to the point where Philip exiled her. Oh, and she simply became crazier as time went on, having Kleopatra and her child killed, as well as Philip’s bastard child (and one of Alexander’s heirs), and his wife, some of these supposedly with her own hands. Meanwhile she and her daughter plotted to take over Epirus and Macedonia respectively, to which Alexander claimed his mother chose well because Macedonians would never consent to be ruled by a woman.
But I digress. Whatever Olympias’ reputation and its impact on Alexander’s legitimacy, Philip recognized Alexander as his son and raised him as heir. Perhaps had Philip lived his son by Kleopatra, Caranus, would have become heir, but then he would have had to compete with Alexander. But that is no more than idle speculation. Alexander had all the prerequisites for a Macedonian king: charisma, skill at arms, skill as a general, and born of the king. Every question about his place in society comes back to two points: Alexander, no matter his mother, was more legitimate than Philip had been and that did not hamper Philip overly much because of his success. Second, Philip died and Alexander, unlike the infant or even unborn Caranus, was mature; he came, he saw, he conquered. Legitimacy stems as much from the subjects as the rulers; to them Alexander was legitimate.