Which ideas are so dangerous they should be kept from the public?

This is a question that occupied Spinoza’s thoughts on a daily base. He was convinced that he and his friends were not safe and had to fear for their lives if his writing would be published under his own name. He was probably right.



In countries like Belarus, Iran and North Korea, but also China and even Western Europe or the US, some ideas are dangerous for those who express them. Apparently rulers regard them as harmful for society. 



Does this mean there’s censorship? Or are there actually ideas best left undiscussed in public at the moment. These days, isn’t it also dependable on the public arena in which the idea arises? 



A quite common idea, like the non-existence of an apparent Ur-Dutchman and that national identity is the sum of total of history, based on internal differences and diversity, becomes a sensitive subject when uttered by crown princess Máxima. You can also state, that when ideas can’t be expressed in any public arena at all, something is wrong with the freedom of the country. 



It’s always people, not books or formulas overthrowing power.