in a way that not only respects the principle of legality, but also ensures that it is restricted to its legitimate purpose. Article 19 of the International Covenant on Civil and Political Rights allows only limitations on the freedom of expression that are “necessary” for the achievement of the purposes listed in its paragraph 3. Prohibiting incitement to terrorism must therefore be limited to what is actually required to protect national security or public order. The provision, and the way in which it is applied, must also be proportional, i.e., for each measure, one must determine whether, given the importance of the right or freedom, the impact of the measure on the enjoyment of that right or freedom is proportional to the importance of the objective being pursued by the measure and its potential effectiveness in achieving that objective.43 The merit of any measure will depend on the importance of the counter-terrorism objective it pursues, as well as on its potential efficacy in achieving it. The imposition of a limitation on rights and freedoms for the purpose of countering terrorism, but by ineffective means, is unlikely to be justifiable. In assessing the impact of a counterterrorism measure on rights and freedoms, consideration must be given, case by case, to the level to which it limits the right or freedom, and also to the importance and degree of protection offered by the human right being limited.44 2.  Derogations In a limited set of circumstances, such as a public emergency which threatens the life of the nation, States may take measures to derogate from certain human rights provisions under the International Covenant on Civil and Political Rights. Its article 4 sets out the formal and substantive requirements which a State party must fulfil to derogate legitimately from certain obligations under the Covenant.45 A state of emergency must be understood as a truly exceptional, temporary measure to which may be resorted only if there is a genuine threat to the life of the nation. Short of such extreme situations, States must develop and implement effective domestic legislation and other measures in compliance with their international human rights obligations. Through the intermediary of the United Nations Secretary-General, a derogating State must immediately inform other States parties to the Covenant of the provisions from which it has derogated and of the reasons for which it has done so. Moreover, the State party must be faced with a situation which constitutes a threat to the life of the nation and may take only such measures as strictly required by the exigencies of that situation. This requirement relates to the degree of interference as well as to the 26