Depends what we mean by post-truth.
The Oxford Dictionary, which chose ‘post-truth’ as the word for the year 2016 defines it thus: “relating to or denoting circumstances in which objective facts are less influential in shaping public opinion than appeals to emotion and personal belief”. That certainly seemed to be the case when it came to the world-changing political developments in the UK and in the US in 2016. We witnessed a situation in which well-established facts were questioned or denied, with little or no evidence, merely for the purposes of promoting a political agenda. What is more, this practice seemed to have the desired effect, mobilizing crowds adhering to the political agendas in question. Such examples included distorting the figure of the amount of money the UK contributes to the EU budget, questioning whether climate change is real, and, more recently, claiming that Trump’s inauguration had a larger audience than that of Obama’s in 2009.
We witnessed a situation in which well-established facts were questioned or denied, with little or no evidence, merely for the purposes of promoting a political agenda.
What is striking about these examples is that they are all claims whose truth should be possible to settle without much difficulty, and so consensus easily arrived at. The reason why people’s beliefs on these matters failed to converge on the truth can be put down to two factors. On the one hand, to make a judgment about the truth of these subject matters, one must rely on the testimony of people with some level of expertise, able to investigate the matters at hand. But experts of all kinds, including scientists and journalists, enjoy low levels of trust from parts of the population, and so their word carries little weight for them. On the other hand, people are prone to believing things that fit-in with a narrative that serves their biases and self-interests, and so are ready to adopt false beliefs, even in the absence of any evidence.
Even if Nietzsche did want to challenge the existence of absolute and universal truth, I doubt Nietzsche would have thought that a simple question such as how many people attended an inauguration had no one true answer.
There is a also a more radical, ‘philosophical’ reading of post-truth, namely the claim that there is no such thing as ‘truth’, only perspectives or interpretations. Such a position is associated with the 19th century philosopher Friedrich Nietzsche, even though his mind on this topic changes depending on which period of his writings we look at. Even if Nietzsche did want to challenge the existence of absolute and universal truth, I doubt Nietzsche would have thought that a simple question such as how many people attended an inauguration had no one true answer. When undermining the existence of universal truths, Nietzsche seems to have had in mind primarily claims about morality. But whether universal moral truths exist is still a question philosophy is grappling with, without at the same time doubting the existence of ‘truth’ altogether. Nietzsche also seemed to question science’s objectivity, declaring that “even physics is only a way of interpreting or arranging the world (if I may say so: according to us!)...” (Beyond Good and Evil, §14). But again, this thought is not of much help to climate change sceptics who question the objectivity of scientists’ reports. Nietzsche is making the old, Kantian, claim that the way we perceive and theorize about the world reflects our human cognitive structures. He is not claiming that because science is best thought of as our interpretation of reality, mediated by the way the human mind is able to comprehend it, one can feel free to dismiss it in favor of an alternative interpretation if they don’t like the results. So as far as Nietzsche’s challenges to truth and objectivity are concerned, they cannot be used to make claims about the dawning of a post-truth era in which there are no universal truths, only interpretations, or sets of ‘alternative facts’.