Article 1, Section 8, Clause 4 (Citizenship)
William Blackstone, Commentaries 1:354, 357--58, 361--621765
The first and most obvious division of the people is into aliens and natural-born subjects. Natural-born subjects are such as are born within the dominions of the crown of England, that is, within the ligeance, or as it is generally called, the allegiance of the king; and aliens, such as are born out of it. Allegiance is the tie, or ligamen, which binds the subject to the king, in return for that protection which the king affords the subject. The thing itself, or substantial part of it, is founded in reason and the nature of government; the name and the form are derived to us from our Gothic ancestors.
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Allegiance, both express and implied, is however distinguished by the law into two sorts or species, the one natural, the other local; the former being also perpetual, the latter temporary. Natural allegiance is such as is due from all men born within the king's dominions immediately upon their birth. For, immediately upon their birth, they are under the king's protection; at a time too, when (during their infancy) they are incapable of protecting themselves. Natural allegiance is therefore a debt of gratitude; which cannot be forfeited, cancelled, or altered, by any change of time, place, or circumstance, nor by any thing but the united concurrence of the legislature. An Englishman who removes to France, or to China, owes the same allegiance to the king of England there as at home, and twenty years hence as well as now. For it is a principle of universal law, that the natural-born subject of one prince cannot by any act of his own, no, not by swearing allegiance to another, put off or discharge his natural allegiance to the former: for this natural allegiance was intrinsic, and primitive, and antecedent to the other; and cannot be devested without the concurrent act of that prince to [Volume 2, Page 558] whom it was first due. Indeed the natural-born subject of one prince, to whom he owes allegiance, may be entangled by subjecting himself absolutely to another; but it is his own act that brings him into these straits and difficulties, of owing service to two masters; and it is unreasonable that, by such voluntary act of his own, he should be able at pleasure to unloose those bands, by which he is connected to his natural prince.
Local allegiance is such as is due from an alien, or stranger born, for so long time as he continues within the king's dominion and protection: and it ceases, the instant such stranger transfers himself from this kingdom to another. Natural allegiance is therefore perpetual, and local temporary only: and that for this reason, evidently founded upon the nature of government; that allegiance is a debt due from the subject, upon an implied contract with the prince, that so long as the one affords protection, so long the other will demean himself faithfully. As therefore the prince is always under a constant tie to protect his natural-born subjects, at all times and in all countries, for this reason their allegiance due to him is equally universal and permanent. But, on the other hand, as the prince affords his protection to an alien, only during his residence in this realm, the allegiance of an alien is confined (in point of time) to the duration of such his residence, and (in point of locality) to the dominions of the British empire.
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When I say, that an alien is one who is born out of the king's dominions, or allegiance, this also must be understood with some restrictions. The common law indeed stood absolutely so; with only a very few exceptions: so that a particular act of parliament became necessary after the restoration, for the naturalization of children of his majesty's English subjects, born in foreign countries during the late troubles. And this maxim of the law proceeded upon a general principle, that every man owes natural allegiance where he is born, and cannot owe two such allegiances, or serve two masters, at once. Yet the children of the king's embassadors born abroad were always held to be natural subjects: for as the father, though in a foreign country, owes not even a local allegiance to the prince to whom he is sent; so, with regard to the son also, he was held (by a kind of postliminium) to be born under the king of England's allegiance, represented by his father, the embassador. To encourage also foreign commerce, it was enacted by statute 25 Edw. III. st. 2. that all children born abroad, provided both their parents were at the time of the birth in allegiance to the king, and the mother had passed the seas by her husband's consent, might inherit as if born in England: and accordingly it hath been so adjudged in behalf of merchants. But by several more modern statutes these restrictions are still farther taken off: so that all children, born out of the king's ligeance, whose fathers were natural-born subjects, are now natural-born subjects themselves, to all intents and purposes, without any exception; unless their said fathers were attainted, or banished beyond sea, for high treason; or were then in the service of a prince at enmity with Great Britain.
The children of aliens, born here in England, are, generally speaking, natural-born subjects, and entitled to all the privileges of such. In which the constitution of France differs from ours; for there, by their jus albinatus, if a child be born of foreign parents, it is an alien.
A denizen is an alien born, but who has obtained ex donatione regis letters patent to make him an English subject: a high and incommunicable branch of the royal prerogative. A denizen is in a kind of middle state between an alien, and natural-born subject, and partakes of both of them. He may take lands by purchase or devise, which an alien may not; but cannot take by inheritance: for his parent, through whom he must claim, being an alien had no inheritable blood, and therefore could convey none to the son. And, upon a like defect of hereditary blood, the issue of a denizen, born before denization, cannot inherit to him; but his issue born after, may. A denizen is not excused from paying the alien's duty, and some other mercantile burthens. And no denizen can be of the privy council, or either house of parliament, or have any office of trust, civil or military, or be capable of any grant from the crown.
Naturalization cannot be performed but by act of parliament: for by this an alien is put in exactly the same state as if he had been born in the king's ligeance; except only that he is incapable, as well as a denizen, of being a member of the privy council, or parliament, &c. No bill for naturalization can be received in either house of parliament, without such disabling clause in it. Neither can any person be naturalized or restored in blood, unless he hath received the sacrament of the Lord's supper within one month before the bringing in of the bill; and unless he also takes the oaths of allegiance and supremacy in the presence of the parliament.
The Founders' Constitution
Volume 2, Article 1, Section 8, Clause 4 (Citizenship), Document 1
The University of Chicago Press
Blackstone, William. Commentaries on the Laws of England: A Facsimile of the First Edition of 1765--1769. Chicago: University of Chicago Press, 1979.