Earlier in his life, Karl Jaspers had loudly advocated resistance to the Nazi regime; in this book, he implicitly reveals his preoccupation with critical exchanges that aim to complexify our views of world history. The big deal with Kant is that he was one of the first philosophers to insist that we learn from each other just as we learn from what we think of or observe in the world.
Introductory philosophy courses will teach that Kant's central contribution is the "categorical imperative," which gives us guidelines of what decisions to make in any situation. The classic example is that if someone comes to your door, claims to want to kill you and your family, and then asks to be let in - conventional politesse instructs us to let someone in the door when they ask to be, but the categorical imperative is to defend human life. Jaspers doesn't seem to understand Kant in this way. His analysis suggests that the categories by which we comprehend the world can enhance the types of judgments which we are able to make.
In a parallel work, Samuel Delaney asks what difference it makes to call an object a tree, a conifer, or a bristlecone pine. Concerned as he is with the interplay between identity and language as it moves in and out of queer communities, Delaney's theory offers the idea that a carpenter, for instance, will require one thing of a pine tree and another thing of an oak; on the contrary, a painter will use a different standard of judgment to decide whether a madrone or a willow will be best for their landscape; at another extreme, a person looking for someplace warm to sleep will be less concerned with what to call a tree than whether it provides enough cover to keep them warm.
Such considerations form only one part of Kant's philosophy. Jaspers is really good at explaining complicated ideas and reminding us that while there will be more to learn, we can do it. He's also skilled at presenting arguments and then showing how wrong they are. It's not like he wants to shoot anybody down, but just points out errors so we can think more clearly.