I appeal to the philosophers of all countries to unite and never again mention Heidegger or talk to another philosopher who defends Heidegger. This m… - Karl Popper

Again, we cannot search the whole world in order to make sure that nothing exists which the law forbids. Nevertheless, both kinds of strict statements, strictly existential and strictly universal, are in principle empirically decidable, each, however, in one way only: they are unilaterally decidable. Whenever it is found that something exists here or there, a strictly existential statement may thereby be verified, or a universal one falsified.

What I criticize under the name Utopian engineering recommends the reconstruction of society as a whole, i.e. very sweeping changes whose practical consequences are hard to calculate, owing to our limited experiences. It claims to plan rationally for the whole of society, although we do not possess anything like the factual knowledge which would be necessary to make good such an ambitious claim.

First, it must be admitted that certain forms of behaviour may be described as more ‘natural’ than other forms; for instance, going naked or eating only raw food; and some people think that this in itself justifies the choice of these forms. But in this sense it certainly is not natural to interest oneself in art, or science, or even in arguments in favour of naturalism. The choice of conformity with ‘nature’ as a supreme standard leads ultimately to consequences which few will be prepared to face; it does not lead to a more natural form of civilization, but to beastliness14. The second criticism is more important. The biological naturalist assumes that he can derive his norms from the natural laws which determine the conditions of health, etc., if he does not naïvely believe that we need adopt no norms whatever but simply live according to the ‘laws of nature’. He overlooks the fact that he makes a choice, a decision; that it is possible that some other people cherish certain things more than their health (for instance, the many who have consciously risked their lives for medical research). And he is therefore mistaken if he believes that he has not made a decision, or that he has derived his norms from biological laws.