The first thing to know is that in English the word "Condescension" is used to mean the OPPOSITE of what it originally meant. Most English speakers do no even understand this. Explanation: Condescension originally meant to lower onesself politely or gracefully. In older books (British literature, usually) you will see the word condescension used as a compliment: "The king was very condescending to his subjects" would mean the king was kind and willing to speak to his subjects as equals. If the most important man in your country spoke to you like a friend, he would be "condescending." That is the OLD definition. No one uses condescension this way anymore. When the United States separated from England, it established a society with the belief that everyone was equal. This is why we do not have nobles, or kings. So the idea of someone "lowering" themself to talk to you was insulting because it presupposed that they were higher than you. So now we use condescending to refer to someone who speaks to you as if you are beneath them, or less important than them. "He is always very condescending to me, speaking to me like I was a fool, or a child." If the most important person in your country spoke to you as if you were unimportant or not worth his time to talk to, he would be condescending. That is how the word is used NOW. The key to remember is that condescension is considered a bad thing. It is insulting to speak "condescendingly" to people because it means you think you are better than them. Posterity = future generations. You children and their children and their children, etc. So "the enormous condescension of posterity" means, "the large amount of negative judgement that the future generations will apply to the actions of the past." When we examine people who lived before us we think they are beneath us, or less intelligent or important than us because we do not understand their way of life. That is the enormous condescension of posterity.