"The ideals in question prominently featured faith in the power of reason. Aron’s discrimination showed itself in his recognition that reason’s power is always

"The ideals in question prominently featured faith in the power of reason. Aron’s discrimination showed itself in his recognition that reason’s power is always limited." That's the key point. There have been two competing traditions in Western philosophy, going back to the Greeks. They have to do with the various finitudes that humans experience, such as ignorance, death, and suffering. According to one view, these are illusions and not only is human understanding potential infinite, but so are all the other types of finitude that interconnect with it. Perhaps the most famous proponent of this view is Plato. The other view, perhaps first made clear by Aristotle, is that human beings and human existence, while they can be improved in important ways, can never be made perfect because our ability to understand the world and act in it are always limited. In the modern era, Marx thought we could abolish most or all finitudes, and have a utopia. Some liberals also think something like this, but the Anglo-American political tradition is more on the Aristotelian side, as was Aron.