In the history of Western thought, from Plato to Positivism, the idea that it was possible to reach an ultimate and definitive truth in both the scientific and the ethical and political realms has prevailed. Alongside this, however, there has always been a minority and generally marginalized current, from the sophists to the nihilists of the 19th century, which denies the possibility of arriving at certain knowledge, asserting that everything is true and everything is false: essentially, we would know nothing, and everything would be a social convention.
Since the late 19th century, the idea has emerged and become predominant that we cannot reach an ultimate and definitive truth even in the field of natural sciences. However, this does not imply the futility of all knowledge, suggesting that everything is true and everything is false in both the scientific and ethical and political fields.
Let's start with scientific thought. Positivism essentially asserted that (natural) science was the only activity capable of attaining certain and indisputable truth through scientific experiment (scientific as a synonym for certain) and that this method could then be applied to the human, ethical, and political fields. The progress of the sciences themselves has invalidated this claim. If classical physics had shown that what seemed entirely evident (e.g., geocentrism) was false, new physics also surpasses those concepts that seemed absolutely certain, starting with special and general relativity and then with quantum physics.
This has led to a new conception of science, according to which a scientific theory is valid if it can be falsified (Popper and others): a scientific law is valid until it is falsified but confirmed by experiments. However, this does not mean that everything is true and everything is false, but rather that the truth of scientific laws is based on experiences conducted so far and might (only might) prove false with further experiences. Therefore, scientific truth is not definitive as it can always be surpassed, but until it is eventually surpassed, it remains true.
We believe a similar discourse can be made for the human field. It is not that in ethics everything is good and everything is evil, and therefore there is neither good nor evil. In reality, even in the ethical sphere, good depends on the principles and values adopted by the community based on the historical moment, the cultural and material context. For example, we read that Muhammad married a 9-year-old girl. For us moderns, he would be an ignoble pedophile, worthy of utmost contempt, but in his time he was considered the messenger of God (prophet, we say) worthy of the highest respect.
But it is not that both judgments are true and therefore neither is. One must consider the historical moment: in those times, a woman was considered ready for marriage as soon as she reached puberty, while for us it is at least 18 years, but generally much later. The difference is not unfounded but arises from the context: in ancient times, every human group was always on the verge of extinction due to very high infant mortality, recurring epidemics, incessant wars. Therefore, it was necessary for a woman to start procreating as soon as she was able, which is what nature expects. In our times, it is thought that a woman should study, integrate into the world of work and society, so marriage occurs much later. If one day low birth rates become an imminent danger, then the role of women might be revised.
This means that every judgment of good and evil depends on the culture which in turn is linked to material conditions. A similar argument can be made for political principles. For example, until a few centuries ago, it was believed that the government should not depend on the people, hence concretely on the majority of the people, but that it was necessary to identify people above the parties with different criteria: from heredity to religious designation to personal abilities and generally a mix of various criteria. From the 19th century onwards, the principle that power belongs to the people who designate their representatives in various ways has prevailed. Even the dictatorships of the last century (fascisms and communisms) actually posed as representatives of the people (of the proletariat and the nation) but not elected in pluralistic electoral competitions. Even in this case, it is not that everything is good and everything is evil but depends on the political conception assumed: exacerbated nationalism or the advent of communism. In democracies, everyone can say anything, but then it must be argued, supported by considerations: in a free and democratic society, a free debate takes place and therefore absurd theories, contrary to reality, gain little support compared to those more substantiated. The idea that Jews were subhumans could assert itself in totalitarian Germany, but in a free society it would immediately appear ridiculous. So it is not that everything is true and everything is false: those ideas that are better argued prevail.
What changes in the scientific field compared to the human field is that in the former only a very limited number of scientists are able to evaluate the experiments and therefore everyone else adapts: it is not that everyone can give their own evaluation on the movements of the planets. Democracy is not foreseen in science. In human fields, however, everyone can express their personal ideas; it is not possible to follow precise procedures accessible only to a few experts and therefore the most logical consequence is the democratic method. If it were possible to have scientific certainties in ethics and politics, democracy would make no sense.