Everything which is not forbidden is allowed

"Everything which is not forbidden is allowed" is a constitutional principle. It is the concept that any action can be taken by an individual or a body unless there is a law against it. It is also known in some situations as the "general power of competence" whereby the body or person being regulated is acknowledged to have competent judgement of their scope of action. The opposite is a principle whereby an action can only be taken if it is specifically allowed. The general power of competence operates in most states and societies as it is much easier to specify what cannot be done, than listing what can be done. 
Individual rights.
In the novel "The Once and Future King", author T. H. White proposed a similarly worded rule as the rule of totalitarianism: "Everything which is not forbidden is compulsory."
White's saying has been used to state the Gell-Mann's totalitarian principle in physics.
A jocular saying is that, in England, "everything which is not forbidden is allowed", while, in Germany, the opposite applies, so "everything which is not allowed is forbidden". This may be extended to France—"everything is allowed even if it is forbidden".
Germany.
The saying about the Germans is at least partially true. In discussion of German Law, an argument often found is that a juristic construction is not applicable since the law doesn't state its existence – even if the law doesn't explicitly state that the construction does not exist. An example for this is the "Nebenbesitz" (indirect possession of a right by more than one person), which is denied by German courts with the argument that §868 of the Bürgerliches Gesetzbuch, which defines indirect possession, doesn't say there could be two people possessing. However, the German constitution Art. 2(1) of the GG protects the general freedom to act ("Allgemeine Handlungsfreiheit"), as demonstrated e.g. by the judgment of the Bundesverfassungsgericht known as “Reiten im Walde” (BVerfGE 80, 137).
Rights of Sovereign States.
International law.
In international law, the principle is known as the Lotus principle, after a collision of the S.S. Lotus in international waters. The Lotus case of 1926–7 established the freedom of sovereign states to act as they wished, unless they chose to bind themselves by a voluntary agreement or there was an explicit restriction in international law.
United Kingdom.
Generally.
Until 2021 it seemed to be generally agreed that the jocular saying referenced above was a true statement of the principle as it applied to the United Kingdom, but in March 2021 in response to the Coronavirus disease 2019 the Health Secretary Matt Hancock reportedly advised the Prime Minister Boris Johnson in the following terms: 'We’ve got to tell people that they can't do anything unless it is explicitly allowed by law.' This advice has been described a 'radical suggestion', and Hancock himself reportedly described it as Napoleonic, "flipping" British tradition, because in Lockdown people would be forbidden from doing anything unless the legislation said, in terms, that they could.' Whilst the foregoing is merely reported the Coronavirus Act 2020 and hundreds of pieces of subordinate legislation made pursuant to that Act prima facie abrogated the principle in the United Kingdom and this has been confirmed by other writers including Adam Wagner, a barrister specialising in human rights and public law. Lord Sumption, a former judge of the Supreme Court, stated in a lecture given on 27 October 2020 that 'The ease with which people could be terrorized into surrendering basic freedoms which are fundamental to our existence as social beings came as a shock to me in March 2020'.
UK Government.
In the United Kingdom, the Ram Doctrine is a constitutional doctrine based upon a 1945 memorandum by Granville Ram. Part of it reads:A Minister of the Crown is not in the same position as a statutory corporation. A statutory corporation (whether constituted by a special statute as, for instance, a railway company is, or constituted under the Companies Acts as in the case of an ordinary company) is entirely a creature of statute and has no powers except those conferred upon it by or under statute, but a Minister of the Crown, even though there may have been a statute authorising his appointment, is not a creature of statute and may, as an agent of the Crown, exercise any powers which the Crown has power to exercise, except so far as he is precluded from doing so by statute. In other words, in the case of a Government Department, one must look at the statutes to see what it may not do, not as in the case of a company to see what it may do.The doctrine is also mentioned in Halsbury's Laws of England (though not explicitly by name) and The Cabinet Manual. 
Case examples:
Criticism:
De Smith's Judicial Review is critical of the doctrine and a 2013 House of Lords Constitution Committee report suggests that Ram's memorandum is not an accurate depiction of the law today and that the phrase "the Ram doctrine" is inaccurate and should no longer be used.
Local authorities in England.
The converse principle—"everything which is not allowed is forbidden"—used to apply to public authorities in England, whose actions were limited to the powers explicitly granted to them by law. The restrictions on local authorities were lifted by the Localism Act 2011 which granted a "general power of competence" to local authorities. 
United States.
Municipal authorities.
In the US, similar restrictions on municipal authorities apply as a consequence of Dillon's rule.
See also.
In Wikimedia and internet context :